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

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.