1. 1 month of age up to < 18 years of age at the time of signing the informed consent. 2. Received HSCT within the past 6 months. 3. Diagnosis of TMA that persists despite initial management of any triggering condition. 4. Body weight ≥ 5 kilograms. 5. Female participants of childbearing potential and male participants with female partners of childbearing potential must use highly effective contraception starting at Screening and continuing until at least 8 months after the last dose of ravulizumab. 6. Participants must be vaccinated against meningococcal infections if clinically feasible, according to institutional guidelines for immune reconstitution after HSCT. Participants must be re-vaccinated against Haemophilus influenzae type b and Streptococcus pneumoniae if clinically feasible, according to institutional guidelines for immune reconstitution after HSCT. All participants should be administered coverage with prophylactic antibiotics according to institutional post-transplant infection prophylaxis guidances, including coverage against Neisseria meningitidis for at least 2 weeks after meningococcal vaccination. Participants who cannot receive meningococcal vaccine should receive antibiotic prophylaxis coverage against Neisseria meningitidis the entire Treatment Period and for 8 months following the final dose of ravulizumab. 1. Known familial or acquired 'a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13' deficiency (activity < 5%). 2. Known Shiga toxin-related hemolytic uremic syndrome. 3. Positive direct Coombs test. 4. Diagnosis or suspicion of disseminated intravascular coagulation. 5. Known bone marrow/graft failure. 6. Diagnosis of veno-occlusive disease (VOD). 7. Human immunodeficiency virus (HIV) infection (evidenced by HIV-1 or HIV-2 antibody titer). 8. Unresolved meningococcal disease. 9. Presence of sepsis requiring vasopressor support. 10. Pregnancy or breastfeeding. 11. Hypersensitivity to murine proteins or to 1 of the excipients of Ravulizumab. 12. Previously or currently treated with a complement inhibitor.

• Patients must be < 50 years at the time of enrollment.
• Patients must have =< 4 nodules per lung consistent with or suspicious for metastases, with at least one of which being >= 3 mm and all of which must be =< 3 cm size.
• Note: Patient must have eligibility confirmed by rapid central imaging review.
• Lung nodules must be considered resectable by either open thoracotomy or thoracoscopic surgery. Determination of resectability is made by the institutional surgeon.
• Patients must have a histological diagnosis of osteosarcoma.
• Patients must have evidence of metastatic lung disease at the time of initial diagnosis, or at time of 1st recurrence following completion of therapy for initially localized disease.
• Patients with newly diagnosed disease must have completed successful gross tumor resection for their primary tumor or surgical local control of primary tumor must be planned to be performed simultaneously with thoracic surgery.
• Newly diagnosed patients must be receiving or recently completed (within 60 days) systemic therapy considered by the treating physician to be standard treatment for newly diagnosed osteosarcoma (eg, cisplatin-doxorubicin or ifosfamide-based drug regimens) at the time of enrollment on this study. Dose and drug modifications for toxicity do not exclude patients from participation.
• Patients at time of 1st recurrence must have completed systemic therapy for their initial primary tumor, considered by the treating physician to be standard treatment for newly diagnosed osteosarcoma (eg, cisplatin-doxorubicin or ifosfamide-based drug regimens) at the time of enrollment on this study. Dose and drug modifications for toxicity do not exclude patients from participation.
• Patients with unresectable primary tumor.
• Patients with pulmonary metastatic lesions that would require anatomic resection (lobectomy or pneumonectomy) or lesions that are defined as "central" (i.e., central lesion involves or is proximal to segmental bronchi and peripheral is lesion distal to segmental bronchi).
• Patients with chest wall or mediastinal based metastatic lesions, or with significant pleural effusion.
• Patients with disease progression at either the primary or pulmonary metastatic site while on initial therapy. Note: Once the patient has been enrolled on the study, additional computed tomography (CT) scans are not anticipated prior to thoracic surgery. Note: Some variation in nodule size measurements over the course of pre-operative therapy is anticipated and does not qualify for exclusion unless deemed true disease progression by the primary treatment team.
• Patients with evidence of extrapulmonary metastatic disease.
• Patients who received therapeutic pulmonary surgery for lung metastasis prior to enrollment.
• All patients and/or their parents or legal guardians must sign a written informed consent.
• All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met.

• >= 4 and < 18 years at time of study entry
• Patients must weigh 15 kg or greater at time of study entry
• Primary central nervous system tumors that have not received prior cranial radiotherapy
• Planned focal, cranial or craniospinal radiation treatment for a primary central nervous system tumor
• The patient must have receptive and expressive language skills in English, French or Spanish since the neurocognitive function and quality of life (QOL) assessment instruments are available in these languages only
• Creatinine clearance or radioisotope glomerular filtration rate (GFR) >= 70 mL/min/1.73 m^2 or a serum creatinine based on age/gender as follows:
• Age: 4 to < 6 years; Maximum serum creatinine (mg/dL): 0.8 male; 0.8 female
• Age: 6 to < 10 years; Maximum serum creatinine (mg/dL): 1 male; 1 female
• Age: 10 to < 13 years; Maximum serum creatinine (mg/dL): 1.2 male; 1.2 female
• Age: 13 to < 16 years; Maximum serum creatinine (mg/dL): 1.5 male; 1.4 female
• Age: >= 16 years; Maximum serum creatinine (mg/dL): 1.7 male; 1.4 female
• Total bilirubin =< 1.5 x upper limit of normal (ULN) for age
• Serum glutamate pyruvate transaminase (SGPT) (alanine aminotransferase [ALT]) =< 135 U/L
• Note: For the purpose of this study, the ULN for SGPT (ALT) has been set to the value of 45 U/L
• The patient must be able to undergo magnetic resonance imaging
• All patients and/or their parents or legal guardians must sign a written informed consent
• All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met
• Life expectancy of less than 18 months
• Pre-existing conditions:
• Any contraindication or allergy to study drug (memantine or placebo)
• Intractable seizures while on adequate anticonvulsant therapy, defined as more than one seizure per month for the past 2 months or since initiating anticonvulsant therapy
• History of neurodevelopmental disorder such as Down syndrome, Fragile X, William's Syndrome, intellectual disability (presumed intelligence quotient [IQ] < 70), etc
• Co-morbid systemic illnesses, psychiatric conditions, social situations, or other severe concurrent disease which, in the judgment of the investigator, would make the patient inappropriate for entry into this study or interfere significantly with the proper assessment of safety and toxicity of the prescribed regimens or would limit compliance with the study requirements
• Patients with a motor, visual, or auditory condition that precludes participation in computerized neurocognitive assessments
• Patients with any medical condition or taking medications that lead to alterations of urine pH towards the alkaline condition (e.g., renal tubular acidosis, carbonic anhydrase inhibitors, sodium bicarbonate)
• Personal history of prior cranial or craniospinal radiotherapy is not allowed
• Note: Prior anti-cancer therapy including surgery, chemotherapy, targeted agents are allowed as per standard of care clinical treatment guidelines
• Female patients who are pregnant are excluded since fetal toxicities and teratogenic effects have been noted for the study drug. A pregnancy test is required for female patients of childbearing potential
• Lactating females who plan to breastfeed their infants
• Sexually active patients of reproductive potential who do not agree to use an effective contraceptive method for the duration of their study participation

• Signed informed consent from patient, legal guardian(s) and/or adolescents obtained in accordance with local regulations. Pediatric patients must provide assent as required by local regulations.
• Age ≥18 years at the time of informed consent, for sarcoma age ≥16 years of age at time of informed consent/assent
• Measurable disease according to RECIST 1.1
• ECOG performance status 0-1
• Expected survival >3 months
• Platelet counts ≥100,000 cells/mm3
• Hemoglobin ≥9 g/dL
• Adequate renal function with serum creatinine ≤1.5 mg/dL or creatinine clearance ≥60mL/min as calculated using the Cockcroft-Gault equation
• Patient willing and able to comply with the trial protocol
• Systemic chemotherapy, radiotherapy, immunotherapy, or major surgery administered within 3 weeks prior to the first planned dosing of the IMP per protocol
• Patients receiving any other investigational therapy for their cancer within 3 weeks prior to the first planned dosing of the IMP per protocol
• Ongoing radiation toxicities from prior RT therapy
• Patients with a diagnosis of autoimmune diseases or immunodeficiencies or documented infection with human immunodeficiency virus (HIV) or hepatitis B or C virus (active)
• Prior treatment with anti-GD2 antibody

• All patients must be enrolled on APEC14B1 (NCT02402244) and consented to the Molecular Characterization Initiative (Part A) prior to enrollment and treatment on ARST2032 (this trial).
• Patients must be =< 21 years at the time of enrollment.
• Patients must have newly diagnosed embryonal rhabdomyosarcoma (ERMS), spindle cell/sclerosing RMS, or FOXO1 fusion negative alveolar rhabdomyosarcoma (ARMS) (institutional FOXO1 fusion results are acceptable). RMS types included under ERMS include those classified in the 1995 International Classification of Rhabdomyosarcoma (ICR) as ERMS (classic, spindle cell, and botryoid variants), which are reclassified in the 2020 World Health Organization (WHO) classification as ERMS (classic, dense and botryoid variants) and spindle cell/sclerosing RMS (encompassing the historical spindle cell ERMS variant and the newly recognized sclerosing RMS variant). Enrollment in APEC14B1 is required for all patients.
• All patients will be evaluated for stage and clinical group. Note that clinical group designation assigned at the time of enrollment on study remains unchanged regardless of any second-look operation that may be performed.
• Patients will be eligible for the very low-risk stratum (Regimen VA) if they have Stage 1, CG I disease.
• Patients will be eligible for the low-risk stratum (Regimen VAC/VA) if they have Stage 1, CG II disease, Stage 2, CG I or II disease, or Stage 1, CG III (orbit only) disease.
• Paratesticular Tumors: Staging ipsilateral retroperitoneal lymph node sampling (SIRLNS) is required for all patients >= 10 years of age with paratesticular tumors who do not have gross nodal involvement on imaging.
• Extremity Tumors: Regional lymph node sampling is required for histologic evaluation in patients with extremity tumors.
• Clinically or radiographically enlarged nodes must be sampled for histologic evaluation.
• Patients must have a Lansky (for patients =< 16 years of age) or Karnofsky (for patients > 16 years of age) performance status score of >= 50. Patients who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing performance score.
• Peripheral absolute neutrophil count (ANC) >= 750/uL (within 7 days prior to enrollment).
• Platelet count >= 75,000/uL (transfusion independent) (within 7 days prior to enrollment).
• Creatinine clearance or radioisotope glomerular filtration rate (GFR) >= 70 mL/min/1.73 m^2 or a serum creatinine (within 7 days prior to enrollment) based on age/gender as follows:
• Age: 1 month to < 6 months; Maximum serum creatinine (mg/dL): 0.4 (male) : 0.4 (female)
• Age: 6 months to < 1 year; Maximum serum creatinine (mg/dL): 0.5 (male) : 0.5 (female)
• Age: 1 to < 2 years; Maximum serum creatinine (mg/dL): 0.6 (male) : 0.6 (female)
• Age: 2 to < 6 years; Maximum serum creatinine (mg/dL): 0.8 (male) : 0.8 (female)
• Age: 6 to < 10 years; Maximum serum creatinine (mg/dL): 1 (male) : 1 (female)
• Age: 10 to < 13 years; Maximum serum creatinine (mg/dL): 1.2 (male) : 1.2 (female)
• Age: 13 to < 16 years; Maximum serum creatinine (mg/dL): 1.5 (male) : 1.4 (female)
• Age >= 16 years; Maximum serum creatinine (mg/dL): 1.7 (male) : 1.4 (female)
• Total bilirubin =< 1.5 x upper limit of normal (ULN) for age (within 7 days prior to enrollment), and
• If there is evidence of biliary obstruction by the tumor, then the total bilirubin must be < 3 x ULN for age.
• Note: For the purpose of this study, the ULN for SGPT (ALT) has been set to the value of 45 U/L.
• Serum glutamate pyruvate transaminase (SGPT) (alanine aminotransferase [ALT]) =< 135 U/L (within 7 days prior to enrollment).
• All patients and/or their parents or legal guardians must sign a written informed consent.
• All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met.
• Patients who have received prior chemotherapy and/or radiation therapy for cancer prior to enrollment. Surgical resection alone of previous cancer(s) is permitted.
• Patients who have received chemotherapy or radiation for non-malignant conditions (e.g., autoimmune diseases) are eligible. Patients must discontinue chemotherapy for non-malignant conditions prior to starting protocol therapy.
• Vincristine is sensitive substrate of the CYP450 3A4 isozyme. Patients must not have received drugs that are moderate to strong CYP3A4 inhibitors and inducers within 7 days prior to study enrollment.
• Patients unable to undergo radiation therapy, if necessary, as specified in the protocol.
• Evidence of uncontrolled infection.
• Female patients who are pregnant since fetal toxicities and teratogenic effects have been noted for several of the study drugs. A pregnancy test is required for female patients of childbearing potential.
• Lactating females who plan to breastfeed their infants.
• Sexually active patients of reproductive potential who have not agreed to use an effective contraceptive method for the duration of their study participation.

• Patients must be >= 3 years and < 30 years at the time of study enrollment
• Patients must be newly diagnosed with localized primary CNS NGGCT of the suprasellar and/or pineal region by pathology and/or serum or cerebrospinal fluid (CSF) elevation of AFP above institutional normal or > 10 ng/mL or human chorionic gonadotropin (hCG) beta > 100 mIU/mL as confirmed by Rapid Central Marker Screening Review on APEC14B1-CNS. Suprasellar, pineal and bifocal tumors are included. (CSF tumor markers and cytology must be within 31 days prior to enrollment and start of protocol therapy [repeat if necessary]. Serum tumor markers, AFP and hCGbeta must be within 7 days prior to enrollment and start of protocol therapy [repeat if necessary]). Basal ganglia or other primary sites are excluded
• Patients with any of the following pathological elements are eligible: endodermal sinus (yolk sac), embryonal carcinoma, choriocarcinoma, malignant/immature teratoma and mixed germ cell tumor (GCT) (i.e., may include some pure germinoma) if malignant elements listed above are present. Patients with only mature teratoma are excluded. Patients with pure germinoma admixed with mature teratoma are excluded (would be eligible for pure germinoma protocols)
• Patients must have a cranial MRI with and without gadolinium at diagnosis/prior to enrollment. If surgical resection is performed, patients must have pre-operative and post operative brain MRI with and without gadolinium. The post operative brain MRI should be obtained within 72 hours of surgery. If patient has a biopsy only, post-operative brain MRI is recommended but not required (within 31 days prior to study enrollment and start of protocol therapy )
• Patients must have a spine MRI with gadolinium obtained at diagnosis/prior to enrollment. Spine MRI with and without gadolinium is recommended (within 31 days prior to study enrollment and start of protocol therapy)
• Lumbar CSF must be obtained prior to study enrollment unless medically contraindicated. If a patient undergoes surgery and lumbar CSF cytology cannot be obtained at the time of surgery, then it should be performed at least 10 days following surgery and prior to study enrollment. False positive cytology can occur within 10 days of surgery
• Patients must have RAPID CENTRAL TUMOR MARKER REVIEW CSF tumor markers obtained prior to enrollment unless medically contraindicated. Ventricular CSF obtained at the time of CSF diversion procedure (if performed) is acceptable for tumor markers but lumbar CSF is preferred. In case CSF diversion and biopsy/surgery are combined, CSF tumor markers should be collected first
• Peripheral absolute neutrophil count (ANC) >= 1000/uL (within 7 days prior to enrollment)
• Platelet count >= 100,000/uL (transfusion independent) (within 7 days prior to enrollment)
• Hemoglobin >= 8.0 g/dL (may receive red blood cell [RBC] transfusions) (within 7 days prior to enrollment)
• Creatinine clearance or radioisotope glomerular filtration rate (GFR) >= 70 mL/min/1.73 m^2 or a serum creatinine based on age/gender as follows (within 7 days prior to enrollment):
• Age: Maximum serum creatinine (mg/dL)
• 3 to < 6 years: 0.8 (male), 0.8 (female)
• 6 to < 10 years: 1 (male), 1 (female)
• 10 to < 13 years: 1.2 (male), 1.2 (female)
• 13 to < 16 years: 1.5 (male), 1.4 (female)
• >= 16 years: male (1.7), 1.4 (female)
• Total bilirubin =< 1.5 x upper limit of normal (ULN) for age (within 7 days prior to enrollment)
• Serum glutamate pyruvate transaminase (SGPT) (alanine aminotransferase [ALT]) =< 135 U/L (within 7 days prior to enrollment)
• Note: For the purpose of this study, the ULN for SGPT (ALT) has been set to the value of 45 U/L
• Central nervous system function defined as:
• Patients with seizure disorder may be enrolled if on anticonvulsants and well controlled
• Patients must not be in status epilepticus, coma or assisted ventilation prior to study enrollment
• Protocol therapy must begin within 31 calendar days of definitive surgery or clinical diagnosis, whichever is later. If a biopsy only was performed, the biopsy date will be considered the date of definitive surgery. For patients who have a biopsy or incomplete resection at diagnosis followed by additional surgery, the date of the last resection will be considered the date of definitive surgery.
• All patients and/or their parents or legal guardians must sign a written informed consent
• All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met
• NEUROCOGNITIVE FUNCTION AND QUALITY OF LIFE ASSESSMENT:
• English-, Spanish-, or French- speaking
• Note: Patients who speak a language other than English, Spanish, or French will be allowed to participate in ACNS2021 but will not complete the neurocognitive and quality of life assessments
• No known history of neurodevelopmental disorder prior to diagnosis of NGGCT (e.g., Down syndrome, fragile X, William syndrome, intellectual disability). Patients with NF1 will be allowed to participate
• Additional eligibility criteria for the COG Standardized Neuropsychological Battery only: must be at a site that has a psychologist to administer the battery
• Note: If not eligible for the COG Standardized Battery, patients should still complete the Behavior Rating Inventory of Executive Function, Second Edition (BRIEF-2), Pediatric Quality of Life Inventory (PedsQL), Adaptive Behavior Assessment System Third Edition (ABAS-3), and Behavior Assessment System for Children, Third Edition (BASC-3) questionnaires
• Patients with tumors located outside the ventricles (i.e., basal ganglia, thalamus)
• Patients with only mature teratoma and non-elevated markers upon tumor sampling at diagnosis
• Patients who have received any prior tumor-directed therapy for their diagnosis of NGGCT other than surgical intervention and corticosteroids
• Patients with metastatic disease (i.e., MRI evaluation, lumbar CSF cytology or intraoperative evidence of dissemination)
• Female patients who are pregnant, since fetal toxicities and teratogenic effects have been noted for several of the study drugs
• Note: Serum and urine pregnancy tests may be falsely positive due to HCGbeta-secreting germ cell tumors. Ensure the patient is not pregnant by institutional standards
• Lactating females who plan to breastfeed their infants
• Sexually active patients of reproductive potential who have not agreed to use an effective contraceptive method for the duration of their study participation

• Age 2-21 years
• Body weight ≥ 10 kg
• Subject has histologically confirmed diagnosis of any one of the following cancers: (A) Synovial Sarcoma, (B) MPNST, (C) Neuroblastoma, or (D) Osteosarcoma
• Must have previously received a systemic chemotherapy
• Measurable disease according to RECIST v1.1 (or INCR, 2017 Neuroblastoma only).
• HLA-A*02 positive
• Tumor shows MAGE-A4 expression confirmed by central laboratory.
• Performance Status: ECOG 0-1 or Lansky Score ≥ 80
• HLA-A*02:05 in either allele; or any A*02 having same protein sequence as HLA-A*02:05
• History of allergic reactions attributed to compounds of similar chemical or biologic composition to fludarabine, cyclophosphamide.
• History of autoimmune or immune mediated disease
• Known central nervous system (CNS) metastases.
• Other prior malignancy that is not considered by the Investigator to be in complete remission
• Clinically significant cardiovascular disease
• Active infection with human immunodeficiency virus, hepatitis B virus, hepatitis C virus, or human T cell leukemia virus
• Pregnant or breastfeeding

• Patient must have measurable disease
• Patient must have an Eastern Cooperative Oncology Group (ECOG) performance status between 0-2 OR patient must have Lansky performance status of >= 50% or Karnofsky performance status of >= 50%
• Patient must be deemed potentially eligible for a ComboMATCH Treatment Trial as assessed by the enrolling provider
• All patients must have sequencing results available from a National Cancer Institute (NCI) credentialed Designated Laboratory (DL)
• Patients must have locally advanced or advanced histologically documented solid tumors requiring therapy and meet one of the following criteria:
• Patients must have progressed on at least one line of standard systemic therapy OR
• Patients whose disease has no standard treatment that has been shown to prolong overall survival
• Patient must meet one of the following requirements:
• Patients 18 years and older who have tumor amenable to minimal risk image-guided or direct vision biopsy and must be willing and able to undergo a tumor biopsy to obtain samples for research if the patient is to enroll in a ComboMATCH treatment trial OR
• Patients 18 years and older who do not have disease that is biopsiable at minimal risk to the patient must confirm availability of an archival tumor tissue specimen for submission for research if the patient enrolls to a ComboMATCH Treatment Trial. This tumor tissue must meet the following criteria:
• Tissue must have been collected within 12 months prior to registration to the EAY191 Registration Trial
• Patient must not have had a Response Evaluation Criteria in Solid Tumors (RECIST) response (complete response [CR] or partial response [PR]) to any intervening therapy after collection of the tissue
• Formalin-fixed paraffin-embedded tumor tissue block(s) or slides must be available OR
• Patients under 18 years old must confirm availability of an archival tumor tissue specimen for submission for research if patient enrolls to a ComboMATCH Treatment Trial. This tumor tissue must meet the following criteria:
• Formalin-fixed paraffin-embedded tumor tissue block(s) or slides must be available
• NOTE: See specific ComboMATCH Treatment Trial protocol for tissue collection and management instructions. Performance of the mandatory research biopsy or submission of pre-trial formalin-fixed paraffin-embedded (FFPE) and collection and submission of the blood specimens for the integrated studies will be performed under the consent authority of the specific treatment trial protocol to which the patient is registered. No procedures to collect specimens for research only are to be performed for patients registered to the EAY191 Registration Trial only
• NOTE: Each ComboMATCH Treatment Trial contains specific eligibility criteria. If patient is found to not be eligible for the assigned ComboMATCH Treatment Trial, indication of ineligibility will trigger re-evaluation and potential assignment to another Treatment Trial

1. Able to understand the study procedures and agree to participate in the study by providing written informed consent (by participant or legally authorized representative), and assent when applicable. 2. Body weight ≥ 10 kg at time of randomization. 3. Histologically diagnosed H3 K27M-mutant diffuse glioma (new diagnosis). Detection of a missense K27M mutation in any histone H3-encoding gene detected by testing of tumor tissue (immunohistochemistry [IHC] or next-generation sequencing [NGS] in a Clinical Laboratory Improvement Amendments [CLIA]-certified or equivalent laboratory). [Site to provide (as available): ≥ 10 unstained formalin-fixed paraffin-embedded (FFPE) slides from tumor tissue.] 4. At least one, high-quality, contrast-enhanced MRI of the brain obtained prior to starting radiotherapy for submission to sponsor's imaging vendor for central read. For participants who had a surgical resection, this scan must be post-resection; for participants who did not have a resection, this scan may be pre- or post-biopsy. 5. At least one, high-quality, contrast-enhanced MRI of the brain obtained 2 to 6 weeks after completion of frontline radiotherapy. If unable to obtain contrast-enhanced imaging due to lack of venous access after multiple attempts, a patient may still be eligible after collection of a nonenhanced MRI of the brain. [Site to also provide all available MRIs completed prior to initiating treatment with study intervention.] 6. Received frontline radiotherapy 1. Initiated radiotherapy within 12 weeks from the initial diagnosis of H3 K27M-mutant diffuse glioma. 2. Completed radiotherapy within 2 to 6 weeks prior to randomization 3. Completed standard fractionated radiotherapy (eg. 54 to 60 Gy in 28 to 33 fractions given over approximately 6 weeks or hypofractionated radiotherapy (eg. 40 Gy in 15 fractions given over approximately 3 weeks). 7. Karnofsky Performance Status or Lansky Performance Status ≥ 70 at time of randomization. 8. Stable or decreasing dose of corticosteroids and anti-seizure medications for 7 days prior to randomization, if applicable. Stable steroid dose is defined as ≤ 2 mg/day increase (based on dexamethasone dose or equivalent dose of an alternative steroid). 1. Primary spinal tumor. 2. Diffuse intrinsic pontine glioma (DIPG), defined as tumors with a pontine epicenter and diffuse involvement of the pons. 3. Evidence of leptomeningeal spread of disease or cerebrospinal fluid dissemination. 4. Any known concurrent malignancy. 5. New lesion(s) outside of the radiation field. 6. Received whole-brain radiotherapy. 7. Received proton therapy for glioma. 8. Use of any of the following treatments within the specified time periods prior to randomization: 1. ONC201 or ONC206 at any time. 2. Systemic bevacizumab (includes biosimilars) at any time since the initial diagnosis of H3 K27M-mutant diffuse glioma. 3. Temozolomide within past 3 weeks. 4. Tumor treating fields at any time. 5. DRD2 antagonist within past 2 weeks. 6. Any investigational therapy within past 4 weeks. 7. Strong CYP3A4 inhibitors within 3 days. 8. Strong CYP3A4 inducers (includes enzyme-inducing antiepileptic drugs) within 2 weeks. 9. Laboratory test results meeting any of the following parameters within 2 weeks prior to randomization: 1. Absolute neutrophil count < 1.0 × 109/L or platelets < 75 × 109/L. 2. Total bilirubin > 1.5 × upper limit of normal (ULN) (participants with Gilbert's syndrome may be included with total bilirubin > 1.5 × ULN if direct bilirubin is ≤ 1.5 × ULN). 3. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) > 2.5 × ULN. 4. Creatinine clearance ≤ 60 mL/min as calculated by the Cockcroft Gault equation (or estimated glomerular filtration rate < 60 mL/min/1.73 m2). 10. QTc > 480 msec (based on mean from triplicate electrocardiograms) during screening. 11. Known hypersensitivity to any excipients used in the study intervention formulation. 12. Pregnant, breastfeeding, or planning to become pregnant while receiving study intervention or within 3 months after the last dose. Participants of childbearing potential must have a negative serum pregnancy test within 72 hours prior to receiving the first dose of study intervention. 13. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection requiring systemic therapy or psychiatric illness/social situations that would limit compliance with study requirements. 14. Any other condition (eg, medical, psychiatric, or social) that, in the opinion of the investigator, may interfere with participant safety or the ability to complete the study according to the protocol.

To be eligible to participate in the randomized trial, an individual must meet all the following criteria: 1. Provision of signed and dated informed consent form for the randomized trial by patient and/or legal guardian. 2. Age ≤25 years old at time of randomized trial consent. 3. Confirmed diagnosis of idiopathic SAA, defined as: 1. Bone marrow cellularity <25%, or <30% hematopoietic cells. 2. Two of three of the following (in peripheral blood): neutrophils <0.5 x 10^9/L, platelets <20 x 10^9/L, absolute reticulocyte count <60 x 10^9/L or hemoglobin <8 g/dL. 4. No suitable fully matched related donor available (minimum 6/6 match for HLA-A and B at intermediate or high resolution and DRB1 at high resolution using DNA based typing). 5. At least 2 unrelated donors noted on NMDP search who are well matched (9/10 or 10/10 for HLA-A, B, C, DRB1, and DQB1 using high resolution). 6. In the treating physician's opinion, no obvious contraindications precluding them from BMT or IST. 1. Presence of Inherited bone marrow failure syndromes (IBMFS). The diagnosis of Fanconi anemia must be excluded by diepoxybutane (DEB) or equivalent testing on peripheral blood or marrow. Telomere length testing should be sent on all patients to exclude Dyskeratosis Congenita (DC), but if results are delayed or unavailable and there are no clinical manifestations of DC, patients may enroll. If patients have clinical characteristics suspicious for Shwachman-Diamond syndrome, this disorder should be excluded by pancreatic isoamylase testing or gene mutation analysis (note: pancreatic isoamylase testing is not useful in children <3). Other testing per center may be performed to exclude IBMFS. 2. Clonal cytogenetic abnormalities or Fluorescence In-Situ Hybridization (FISH) pattern consistent with pre- myelodysplastic syndrome (pre-MDS) or MDS on marrow examination. 3. Known severe allergy to ATG. 4. Prior allogeneic or autologous stem cell transplant. 5. Prior solid organ transplant. 6. Infection with human immunodeficiency virus (HIV). 7. Active Hepatitis B or C. This only needs to be excluded in patients where there is clinical suspicion of hepatitis (e.g., elevated LFTs). 8. Female patients who are pregnant or breast-feeding. 9. Prior malignancies except resected basal cell carcinoma or treated cervical carcinoma in situ. 10. Disease modifying treatment prior to study enrollment, including but not limited to use of androgens, eltrombopag, romiplostim, or immune suppression. Note: Supportive care measures such as G-CSF, blood transfusion support and antibiotics are allowable

• Patients must be 5 to 60 years of age at the time of enrollment
• Patients with newly diagnosed untreated histologically confirmed classic Hodgkin lymphoma (cHL) (nodular sclerosis, mixed cellularity, lymphocyte-rich, or lymphocyte-depleted, or not otherwise specified [NOS]) with stage I or II disease
• Patients must have bidimensionally measurable disease (at least one lesion with longest diameter >= 1.5 cm)
• Patients must have a whole body or limited whole body PET scan performed within 42 days prior to enrollment. PET-CT is strongly preferred. PET-MRI allowed if intravenous contrast enhanced CT is also obtained
• Pediatric patients (age 5-17 years) must have an upright posteroanterior (PA) chest X-ray (CXR) for assessment of bulky mediastinal disease. Adult patients must have either a CXR or CT chest
• Patients >= 18 years must have a performance status corresponding to Zubrod scores of 0, 1 or 2
• Patients =< 17 years of age must have a Lansky performance score of >= 50
• Pediatric patients (age 5-17 years): A serum creatinine based on age/gender as follows (within 7 days prior to enrollment):
• 2 to < 6 years (age): 0.8 mg/dL (male), 0.8 mg/dL (female)
• 6 to < 10 years (age): 1 mg/dL (male), 1 mg/dL (female)
• 10 to < 13 years (age): 1.2 mg/dL (male), 1.2 mg/dL (female)
• 13 to < 16 years (age): 1.5 mg/dL (male), 1.4 mg/dL (female)
• >= 16 years (age): 1.7 mg/dL (male), 1.4 mg/dL (female) OR a 24 hour urine creatinine clearance >= 50 mL/min/1.73 m^2 (within 7 days prior to enrollment) OR a glomerular filtration rate (GFR) >= 50 mL/min/1.73 m^2 (within 7 days prior to enrollment). GFR must be performed using direct measurement with a nuclear blood sampling method OR direct small molecule clearance method (iothalamate or other molecule per institutional standard)
• Note: Estimated GFR (eGFR) from serum or plasma creatinine, cystatin C or other estimates are not acceptable for determining eligibility
• For adult patients (age 18 years or older) (within 7 days prior to enrollment): Creatinine clearance >= 30 mL/min, as estimated by the Cockcroft and Gault formula or a 24-hour urine collection. The creatinine value used in the calculation must have been obtained within 28 days prior to registration. Estimated creatinine clearance is based on actual body weight
• Total bilirubin =< 2 x upper limit of normal (ULN) (within 7 days prior to enrollment)
• Unless due to Gilbert's disease, lymphomatous involvement of liver or vanishing bile duct syndrome
• Aspartate aminotransferase (AST) =< 3 x ULN (within 7 days prior to enrollment)
• Unless due to Gilbert's disease, lymphomatous involvement of liver or vanishing bile duct syndrome
• Alanine aminotransferase (ALT) =< 3 x ULN (within 7 days prior to enrollment)
• Unless due to Gilbert's disease, lymphomatous involvement of liver or vanishing bile duct syndrome
• Shortening fraction of >= 27% by echocardiogram (ECHO), multigated acquisition scan (MUGA), or functional cardiac imaging scan (within 7 days prior to enrollment) or ejection fraction of >= 50% by radionuclide angiogram, ECHO, MUGA, or cardiac imaging scan (within 7 days prior to enrollment)
• Diffusion capacity of the lung for carbon monoxide (DLCO) >= 50% of predicted value as corrected for hemoglobin by pulmonary function test (PFT) (within 7 days prior to enrollment). If unable to obtain PFTs, the criterion is: a pulse oximetry reading of > 92% on room air
• Known human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial
• For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral load must be undetectable on suppressive therapy, if indicated. Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load
• Patients with nodular lymphocyte predominant Hodgkin lymphoma
• Patients with a history of active interstitial pneumonitis or interstitial lung disease
• Patients with a diagnosis of inherited or acquired immunodeficiency that is poorly controlled or requiring active medications, such as primary immunodeficiency syndromes or organ transplant recipients
• Patients with any known uncontrolled intercurrent illness that would jeopardize the patient's safety such as infection, autoimmune conditions, cardiac arrhythmias, angina pectoris, and gastrointestinal disorders affecting swallowing and/or absorption of pills
• Patients with a condition requiring systemic treatment with either corticosteroids (defined as equivalent to > 10 mg daily prednisone for patients >= 18 years or > 0.5 mg/kg [up to 10 mg/day] for patients < 18 years) or other immunosuppressive medications within 14 days prior to enrollment
• Note: Replacement therapy such as thyroxine, insulin, or physiologic corticosteroid for adrenal or pituitary insufficiency is not considered a form of systemic treatment. Inhaled or topical steroids, and adrenal replacement doses (=< 10 mg daily for patients >= 18 years or =< 0.5 mg/kg [up to 10 mg/day] prednisone equivalents) are permitted in the absence of active autoimmune disease
• Note: Steroid use for the control of Hodgkin lymphoma symptoms is allowable, but must be discontinued by cycle 1, day 1
• Patients with peripheral neuropathy > grade 1 at the time of enrollment or patients with known Charcot-Marie-Tooth syndrome
• Patients with a prior or concurrent malignancy whose natural history or treatment has the potential to interfere with the safety or efficacy assessment of the investigational regimen
• Administration of prior chemotherapy, radiation, or antibody-based treatment for cHL
• Prior solid organ transplant
• Prior allogeneic stem cell transplantation
• Live vaccine within 30 days prior to planned day 1 of protocol therapy (e.g., measles, mumps, rubella, varicella, yellow fever, rabies, bacillus calmette guerin [BCG], oral polio vaccine, and oral typhoid). Administration of messenger ribonucleic acid (mRNA) vaccines are permitted
• Female patients who are pregnant since fetal toxicities and teratogenic effects have been noted for several of the study drugs. A pregnancy test within 28 days prior to enrollment is required for female patients of childbearing potential
• Lactating females who plan to breastfeed their infants starting with the first dose of study therapy and for at least 6 months after the last treatment
• Sexually active patients of reproductive potential who have not agreed to use a highly effective contraceptive method (failure rate of < 1% per year when used consistently and correctly) for the duration of their study drug therapy. Following therapy, patients will be advised to use contraception as per institutional practice or as listed below for investigational agents, whichever is longer
• Men and women of childbearing potential must continue contraception for a period of 6 months after last dose of brentuximab vedotin
• Women of child-bearing potential (WOCBP) must continue contraception for a period of at least 5 months after the last dose of nivolumab
• All patients and/or their parents or legal guardians must sign a written informed consent
• All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met

• PATIENT INCLUSION CRITERIA FOR ENROLLMENT:
• 6 months to < 22 years at enrollment
• Diagnosed with ALL, AML, or MDS or mixed phenotype acute leukemia (MPAL) for which an allogeneic hematopoietic stem cell transplant is indicated. Complete Remission (CR) status will not be confirmed at the time of enrollment. CR as defined in these sections is required to proceed with the actual HCT treatment plan
• Has not received a prior allogeneic hematopoietic stem cell transplant
• Does not have a suitable human leukocyte antigen (HLA)-matched sibling donor available for stem cell donation
• Has an eligible haploidentical related family donor based on at least intermediate resolution HLA typing
• Patients who also have an eligible 8/8 MUD adult donor based on confirmatory high resolution HLA typing are eligible for randomization to Arm A or Arm B.
• Patients who do not have an eligible MUD donor are eligible for enrollment to Arm C
• All patients and/or their parents or legal guardians must sign a written informed consent
• All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met
• Co-Enrollment on other trials
• Patients will not be excluded from enrollment on this study if already enrolled on other protocols for treatment of high risk and/or relapsed ALL, AML, MPAL and MDS. This is including, but not limited to, COG AAML1831, COG AALL1821, the EndRAD Trial, as well as local institutional trials. We will collect information on all co-enrollments
• Patients will not be excluded from enrollment on this study if receiving immunotherapy prior to transplant as a way to achieve remission and bridge to transplant. This includes chimeric antigen receptor (CAR) T cell therapy and other immunotherapies
• PATIENT INCLUSION CRITERIA TO PROCEED TO HCT:
• Karnofsky Index or Lansky Play-Performance Scale >= 60 on pre-transplant evaluation. Karnofsky scores must be used for patients >= 16 years of age and Lansky scores for patients =< 16 years of age (within 4 weeks of starting therapy)
• A serum creatinine based on age/gender as follows: 6 months to < 1 year: 0.5 mg/dL (Male); 0.5 mg/dL (Female) 1. to < 2 years: 0.6 mg/dL (Male); 0.6 mg/dL (Female) 2. to < 6 years: 0.8 mg/dL (Male); 0.8 mg/dL (Female) 6 to < 10 years: 1 mg/dL (Male); 1 mg/dL (Female) 10 to < 13 years: 1.2 mg/dL (Male); 1.2 mg/dL (Female) 13 to < 16 years: 1.5 mg/dL (Male); 1.4 mg/dL (Female) >= 16 years: 1.7 mg/dL (Male); 1.4 mg/dL (Female)
• OR
• A 24 hour urine Creatinine clearance >= 60 mL/min/1.73 m^2
• OR
• A glomerular filtration rate (GFR) >= 60 mL/min/1.73 m^2. GFR must be performed using direct measurement with a nuclear blood sampling method OR direct small molecule clearance method (iothalamate or other molecule per institutional standard)
• Note: Estimated GFR (eGFR) from serum creatinine, cystatin C or other estimates are not acceptable for determining eligibility
• Serum glutamic-oxaloacetic transaminase (SGOT) aspartate aminotransferase [AST] or serum glutamate pyruvate transaminase (SGPT) aminotransferase [ALT] < 5 x upper limit of normal (ULN) for age
• Total bilirubin < 2.5 mg/dL, unless attributable to Gilbert's Syndrome
• Shortening fraction of >= 27% by echocardiogram or radionuclide scan (MUGA)
• OR
• Ejection fraction of >= 50% by echocardiogram or radionuclide scan (MUGA), choice of test according to local standard of care
• Forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and corrected carbon monoxide diffusing capability (DLCO) must all be >= 50% of predicted by pulmonary function tests (PFTs).
• For children who are unable to perform for PFTs (e.g., due to age or developmental delay), the criteria are: no evidence of dyspnea at rest, oxygen (O2) saturation (Sat) > 92% on room air by pulse oximetry, not on supplemental O2 at rest, and not on supplemental O2 at rest
• MPAL in first complete remission (CR1) for whom transplant is indicated. Examples include those patients who are poorly responsive to ALL therapy (end of induction failure( IF-MPAL) to ALL induction (see IF-MPAL note below), end of induction MRD ≥ 5% or end-of-consolidation MRD > 0.01%), as well as patients treated with AML therapy
• IF-MPAL: additional criterion for Induction failure for MPAL ONLY as per ALL1732:
• An increasing number of circulating leukemia cells on 3 or more consecutive CBCs obtained at daily or longer intervals following day 8 of Induction therapy and prior to day 29 with confirmation by flow cytometry OR development of new sites of extramedullary disease, or other laboratory or clinical evidence of refractory disease or progression prior to the end of Induction evaluation (note that residual testicular disease at the end of Induction is an exception)
• MPAL in > second complete remission (CR2)
• ALL high-risk in CR1 for whom transplant is indicated. Examples include: induction failure, treatment failure as per minimal residual disease by flow cytometry > 0.01% after consolidation and not eligible for AALL1721 or AALL1721 not available/unwilling to enroll, hypodiploidy (< 44 chromosomes) with MRD+ > 0.01% after induction, persistent or recurrent cytogenetic or molecular evidence of disease during therapy requiring additional therapy after induction to achieve remission (e.g. persistent molecular BCR-ABL positivity), T cell ALL with persistent MRD > 0.01% after consolidation.
• ALL in CR2 for whom transplant is indicated. Examples include: B-cell: early (=< 36 months from initiation of therapy) bone marrow (BM) relapse, late BM relapse (>= 36 months) with MRD >= 0.1% by flow cytometry after first re-induction therapy; T or B-cell: early (< 18 months) isolated extramedullary (IEM), late (>= 18 months) IEM, end-Block 1 MRD >= 0.1%; T-cell or Philadelphia chromosome positive (Ph+): BM relapse at any time
• ALL in >= third complete remission (CR3)
• Patients treated with chimeric antigen receptor T-cells (CART) cells for whom transplant is indicated. Examples include: transplant for consolidation of CART, loss of CART persistence and/or B cell aplasia < 6 months from infusion or have other evidence (e.g., MRD+) that transplant is indicated to prevent relapse
• AML in CR1 for whom transplant is indicated. Examples include those deemed high risk for relapse as described in AAML1831:
• FLT3/ITD+ with allelic ratio > 0.1 without bZIP CEBPA, NPM1
• FLT3/ITD+ with allelic ratio > 0.1 with concurrent bZIP CEBPA or NPM1 and with evidence of residual AML (MRD >= 0.05%) at end of Induction
• Presence of RAM phenotype or unfavorable prognostic markers (other than FLT3/ITD) per cytogenetics, fluorescence in situ hybridization (FISH), next generation sequencing (NGS) results, regardless of favorable genetic markers, MRD status or FLT3/ITD mutation status
• AML without favorable or unfavorable cytogenetic or molecular features but with evidence of residual AML (MRD >= 0.05%) at end of Induction
• Presence of a non-ITD FLT3 activating mutation and positive MRD (>= 0.05%) at end of Induction 1 regardless of presence of favorable genetic markers.
• AML in >= CR2
• MDS with < 5% blasts by morphology and flow cytometry (if available) on the pre-transplant bone marrow evaluation
• Complete remission (CR) is defined as < 5% blasts by morphology and flow cytometry (if available) on the pre-transplant bone marrow evaluation with minimum sustained absolute neutrophil count (ANC) of 300 cells/microliter for 1 week or ANC > 500 cells/microliter. We will be collecting data from all approaches to MRD evaluation performed including NGS and polymerase chain reaction (PCR). It is strongly recommended that MPAL be evaluated using multidimensional flow cytometry and/or (KMT2Ar) qt PCR. It is strongly recommended that MPAL be evaluated using multidimensional flow cytometry and/or (KMT2Ar) qt PCR
• DONOR ELIGIBILITY CRITERIA:
• Matched Unrelated Donors: Unrelated donor candidates must be matched at high resolution at a minimum of 8/8 alleles (HLA-A, -B, -C, -DRB1). One-antigen HLA mismatches are not permitted. HLA matching of additional alleles is recommended according to National Marrow Donor Program (NMDP) guidelines, but will be at the discretion of local centers
• Haploidentical Matched Family Members:
• Minimum match level full haploidentical (at least 5/10; HLA-A, -B, -C, -DRB1, -DQB1 alleles). The following issues (in no particular order) should be considered in choosing a haploidentical donor:
• Absent or low patient donor-specific antibodies (DSA)
• Mean fluorescence intensity (MFI) of any anti-donor HLA antibody by solid phase immunoassay should be < 2000. Donors with higher levels are not eligible.
• If a screening assay against pooled HLA antigens is used, positive results must be followed with specificity testing using a single antigen assay. The MFI must be < 2000 unless the laboratory has validated higher threshold values for reactivity for HLA antigens (such as HLA-C, -DQ, and -DP), that may be enhanced in concentration on the single antigen assays. Donor anti- recipient antibodies are of unknown clinical significance and do not need to be sent or reported.
• Consult with Study Chair for the clinical significance of any recipient anti-donor HLA antibody.
• If centers are unable to perform this type of testing, please contact the Study Chair to make arrangements for testing.
• If killer immunoglobulin testing (KIR) is performed: KIR status by mismatch, KIR-B, or KIR content criteria can be used according to institutional guidelines.
• ABO compatibility (in order of priority):
• Compatible or minor ABO incompatibility
• Major ABO incompatibility
• CMV serostatus:
• For a CMV seronegative recipient: the priority is to use a CMV seronegative donor when feasible
• For a CMV seropositive recipient: the priority is to use a CMV seropositive donor when feasible
• Age: younger donors including siblings/half-siblings, and second degree relatives (aunts, uncles, cousins) are recommended, even if < 18 years
• Size and vascular access appropriate by center standard for peripheral blood stem cell (PBSC) collection if needed
• Haploidentical matched family members: screened by center health screens and found to be eligible
• Unrelated donors: meet eligibility criteria as defined by the NMDP or other unrelated donor registries. If the donor does not meet the registry eligibility criteria but an acceptable eligibility waiver is completed and signed per registry guidelines, the donor will be considered eligible for this study
• Human immunodeficiency virus (HIV) negative
• Not pregnant
• MUD donors and post-transplant cyclophosphamide haplo donors should be asked to provide BM. If donors refuse and other donors are not available, PBSC is allowed. TCR-alpha beta/CD19 depleted haplo donors must agree to donate PBSC
• Must give informed consent:
• Haploidentical matched family members: Institution standard of care donor consent and Protocol-specific Donor Consent for Optional Studies
• Unrelated donors: standard NMDP Unrelated Donor Consent
• PATIENT EXCLUSION CRITERIA FOR ENROLLMENT:
• Patients with genetic disorders (generally marrow failure syndromes) prone to secondary AML/ALL/MPAL with known poor outcomes because of sensitivity to alkylator therapy and/or TBI are not eligible (Fanconi Anemia, Kostmann Syndrome, Dyskeratosis Congenita, etc). Patients with Downs syndrome because of increased toxicity with intensive conditioning regimens.
• Patients with any obvious contraindication to myeloablative HCT at the time of enrollment
• Female patients who are pregnant are ineligible as many of the medications used in this protocol could be harmful to unborn children and infants
• Sexually active patients of reproductive potential who have not agreed to use an effective contraceptive method for the duration of their study participation
• PATIENT EXCLUSION CRITERIA TO PROCEED TO HCT:
• Patients with uncontrolled fungal, bacterial, viral, or parasitic infections are excluded. Patients with history of fungal disease during chemotherapy may proceed if they have a significant response to antifungal therapy with no or minimal evidence of disease remaining by computed tomography (CT) evaluation
• Patients with active central nervous system (CNS) leukemia or any other active site of extramedullary disease at the time of initiation of the conditioning regimen are not permitted.
• Note: Those with prior history of CNS or extramedullary disease, but with no active disease at the time of pre-transplant workup, are eligible
• Pregnant or breastfeeding females are ineligible as many of the medications used in this protocol could be harmful to unborn children and infants