Study to Evaluate Biological & Clinical Effects of Significantly Corrected CFTR Function in Infants & Young Children

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.