This is a two-part, multi-center, prospective longitudinal, exploratory study of highly effective cystic fibrosis transmembrane conductance regulator (CFTR) modulators and their impact on children with cystic fibrosis (CF).

This is a two-part, multi-center, prospective longitudinal, exploratory study of highly effective cystic fibrosis transmembrane conductance regulator (CFTR) modulators and their impact in children with cystic fibrosis (CF) on endocrine growth factors and height, gastrointestinal function and gut microbiome, lung function and respiratory microbiome, liver and pancreatic function, sweat chloride, inflammatory markers, and bone health.

Total duration of the study is expected to be 10 years. Part A will be a prospective cross-sequential study to describe the natural history of hormonal growth factors in early childhood and assess the feasibility of additional measurements. In Part A, subjects will have up to 8 visits over a period of up to 5 years.

Part B will be a prospective longitudinal study to observe the effects of administration of either ivacaftor or elexacaftor/tezacaftor/ivacaftor (elex/tez/iva) on growth. In Part B, subjects will have 1 "before ivacaftor or elex/tez/iva" visit within 30 days before initiation of the therapy and 8 "after ivacaftor or elex/tez/iva" visits over a 60-month follow-up period.

Inclusion Criteria:

• Part A:

  • Less than 10 years of age at the first study visit.
  • Documentation of a CF diagnosis.

Part B:

• Participated in Part A OR less than 7 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.

Exclusion Criteria:

• 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 28 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.

Part A - children with confirmed diagnosis of cystic fibrosis who are less than 10 years of age and not on ivacaftor or elexacaftor/tezacaftor/ivacaftor CFTR modulator therapy

Part B - children with confirmed diagnosis of cystic fibrosis who are less than 7 years of age (or participated in Part A of the study) with an intention to start ivacaftor or elexacaftor/tezacaftor/ivacaftor CFTR modulator therapy