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

Who May Qualify: - 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. Who Should NOT Join This Trial: - 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. Always talk to your doctor about whether this trial is right for you.