This is a prospective, multi-center observational study. The study is designed to measure the clinical effectiveness of elexacaftor, tezacaftor and ivacaftor (ETI) triple combination therapy in people with one or more copies of the F508del mutation, study the effects of ETI across a number of CF disease manifestations, and collect specimens for future research. Subjects in the study will have one "before TCT" visit within 30 days before initiation of the therapy and five "after TCT" visits over a 30-month follow-up period. Participants who have participated in the original PROMISE cohort have the option of participating in a long-term extension with annual visits performed at the 42- and 54-month timepoints. The durability of the clinical and biological changes in PROMISE can be assessed with extended follow-up, which would enable the sub-studies to consider potential clinical consequences of the biological or physiological effects being studied. This work will help to inform long term prognosis and feasibility of certain clinical trials outcomes for interventional studies and may be useful when considering research priorities in drug development. Most participating sites have been divided into sub-study groups; each sub-study group has specific non-optional procedures conducted in addition to the "Core" procedures. Finally, there is one optional procedure (transient elastography) that will be offered to subjects at certain sites. The duration of participation for each subject is 30 months (with an additional 24 months if participants agree to the optional long-term extension). NOTE: FDA has reviewed the New Drug Application (NDA) for elexacaftor, tezacaftor and ivacaftor and has granted approval.

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