1. Written informed consent (and assent when applicable) obtained from subject or subject's legal representative. 2. Willingness and ability to adhere to the study visit schedule and other protocol requirements. 3. Documentation of a CF diagnosis as evidenced by one or more clinical features consistent with the CF phenotype and one or more of the following criteria: 1. Sweat chloride equal to or greater than 60 mEq/liter by quantitative pilocarpine iontophoresis test 2. Two well-characterized mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene 4. Phase 1 only: Age 6 to 18 years, inclusive, at the time of consent. 5. Phase 2 only: Ages 9 to 18 years, inclusive, at the time of consent. 6. Clinically stable with no acute antibiotic usage in the 14 days prior to the first visit. 7. Genotype with F508del on at least one allele. 8. No change in chronic pulmonary medications or therapies in the 28 days prior to the first visit. 9. Stable CFTR modulator therapy (TEZ/IVA or LUM/IVA) for at least 28 days prior to the first visit or currently not receiving CFTR modulator therapy. 10. Ability to cooperate with MRI procedures. 11. Phase 1 only: FEV1 greater than or equal to 80% predicted based on GLI reference equations. 1. Standard MRI exclusions (Metal implants, claustrophobia). 2. For females of childbearing potential: Positive urine pregnancy test at Screening or Visit 1 or Lactating. 3. Any other condition that, in the opinion of the Site Investigator/designee, would preclude informed consent or assent, make study participation unsafe, complicate interpretation of study outcome data, or otherwise interfere with achieving the study objectives.

1. Written informed consent (and assent when applicable) obtained from subject or subject's legal representative 2. Enrolled in the CFFPR 3. Male or female ≥ 4 months of age on day of study visit 4. Diagnosis of CF. 5. Current treatment with a prescribed commercially approved CFTR modulator for at least 90 days prior to enrollment 6. Able to perform the testing and procedures required for this study, as judged by the investigator Additional Inclusion Criteria for CHEC-PKPD Sub-Study: 1. Male or female ≥ 6 years of age on day of study visit. 2. Current treatment with elexacaftor/tezacaftor/ivacaftor for at least 90 days prior to enrollment. 3. Last dose of elexacaftor/tezacaftor/ivacaftor taken at least 24hours and last dose of ivacaftor taken at least 12 hours prior to trough blood draw on day of visit. 1. Presence of a condition or abnormality that, in the opinion of the Investigator, would compromise the safety of the patient or the quality of the data 2. Currently enrolled in an investigational trial (including open-label follow-on studies and Early Access Programs (EAP) of an agent expected to have an impact on sweat chloride (refer to current list provided on study website)

Ivacaftor Arm: Subjects From Study 124 (NCT02725567 ) Part B:
• Subjects transitioning from Study 124 Part B must have completed the last study visit of Study 124 Part B.
• As judged by the investigator, parent or legal guardian must be able to understand protocol requirements, restrictions, and instructions; and must sign the informed consent form (ICF). Ivacaftor Arm: Subjects Not From Study 124 Part B:
• Confirmed diagnosis of CF, or 2 CF-causing mutations.
• An ivacaftor- responsive CFTR mutation on at least 1 allele. Subjects will be eligible in countries/regions where ivacaftor is approved for use in subjects 2 years of age and older.
• As judged by the investigator, parent or legal guardian must be able to understand protocol requirements, restrictions, and instructions; and must sign the ICF. Observational Arm:
• Received ivacaftor treatment in Study 124 Part B and elected not to enroll or ineligible to enroll in the ivacaftor arm of Study 126. Ivacaftor Arm: Subjects From Study 124 Part B:
• History of any illness or condition that, in the opinion of the investigator, might confound the results of the study or pose an additional risk in administering ivacaftor to the subject.
• Subjects receiving commercially available ivacaftor treatment Ivacaftor Arm: Subjects Not From Study 124 Part B:
• History of any illness or condition that, in the opinion of the investigator, might confound the results of the study or pose an additional risk in administering ivacaftor to the subject
• An acute upper or lower respiratory infection, or pulmonary exacerbation, or changes in therapy for pulmonary disease within 4 weeks of Day 1
• Abnormal liver function at screening
• Hemoglobin <9.5 g/dL at screening
• History of solid organ or hematological transplantation
• Use of any moderate or strong inducers or inhibitors of CYP3A within 2 weeks of Day 1 Observational Arm:
• Receiving ivacaftor treatment Other protocol defined Inclusion/Exclusion criteria may apply.

• Part A:
• Less than 5 years of age at the first study visit.
• Documentation of a CF diagnosis. Part B:
• Participated in Part A OR less than 6 years of age at the first study visit.
• Documentation of a CF diagnosis.
• CFTR mutations consistent with FDA labeled indication of highly effective modulator therapy (ivacaftor or elexacaftor/tezacaftor/ivacaftor).
• Physician intent to prescribe ivacaftor or elexacaftor/tezacaftor/ivacaftor.
• Part A and Part B: Use of an investigational drug within 28 days prior to and including the first study visit. Use of ivacaftor or elexacaftor/tezacaftor/ivacaftor within the 180 days prior to and including the first study visit. Use of chronic oral corticosteroids within the 28 days prior to and including the first study visit.

• Pregnant, intending to continue pregnancy, enrolled in the Cystic Fibrosis Foundation Patient Registry (CFFPR)
• None

Key
• Participant has one of the following genotypes:
• Homozygous for F508del;
• Heterozygous for F508del and a gating (F/G) mutation;
• Heterozygous for F508del and a residual function (F/RF) mutation;
• At least 1 other TCR CFTR gene mutation identified as responsive to ELX/TEZ/IVA and no F508del mutation
• Forced expiratory volume in 1 second (FEV1) value >=40% and <=90% of predicted mean for age, sex, and height for participants currently receiving CFTR protein modulator therapy; FEV1 >=40% and <=80% for participants not currently receiving CFTR protein modulator therapy Key
• History of solid organ or hematological transplantation
• Hepatic cirrhosis with portal hypertension, moderate hepatic impairment (Child Pugh Score 7 to 9), or severe hepatic impairment (Child Pugh Score 10 to 15)
• Lung infection with organisms associated with a more rapid decline in pulmonary status
• Pregnant or breast-feeding females Other protocol defined Inclusion/Exclusion criteria may apply