The primary aim is to determine the mechanism(s) for weight gain in participants whom Ivacaftor treatment was initiated based on clinical indications by CF Care Team. This longitudinal study will assess in detail energy expenditure, weight gain, body composition, and lung function in 24 subjects ≥6 years old with CF with a gating mutation before treatment and after three months treatment with Ivacaftor. All subjects will be seen at the Children's Hospital of Philadelphia's Clinical Translational Research Center.