The purpose of this observational research study is to determine the effects of clinically prescribed Orkambi treatment on 2 to 5 year old children homozygous for the F508del Mutations in the Cystic fibrosis transmembrane conductance regulator (CFTR) gene on sleeping energy expenditure, growth status and gut health and function.

Orkambi is a novel FDA approved (August, 2018) therapy for use in patients with cystic fibrosis (CF) who are 2 to 5 years of age and homozygous for F508del mutations in the CFTR gene. It is a combination of lumacaftor and ivacaftor that addresses both the processing and gating defects of the F508del mutation. This investigator-initiated study is designed to evaluate the nutritional, growth and GI impact of Orkambi treatment for this unique younger (2 to 5 years) patient cohort. This proposal extends previous highly informative nutrition and weight gain investigation of ivacaftor treatment in people with CF gating mutations to another CFTR modulator treatment (Orkambi) in people homozygous for F508del mutations. The primary aims of the study are to evaluate the impact of 24 weeks of Orkambi treatment in 2 to 5 year old subjects with CF homozygous for F508del mutations on sleeping or resting energy expenditure, growth status and gut health and function in n=32 children ages 2.0 to 5.9 years of age. Protocol evaluations will occur at baseline (pre-treatment) and 12 and 24 weeks after clinically prescribed Orkambi treatment has begun. Other outcomes of significant clinical interest in young subjects with CF will be explored. All subjects will be evaluated as outpatient at The Children's Hospital of Philadelphia, and will be recruited both regionally and nationally to ensure timely enrollment.

Inclusion Criteria:

• Cystic fibrosis and homozygous for F508del mutations, approved for treatment
• Age: 2.0 to 5.9 years
• In usual state of good health
• A clinical decision has been made for subject to begin Orkambi treatment
• Family committed to the 6 to 8 month study protocol with visits to the Children's Hospital of Philadelphia (CHOP) that will last 2-3 days for the baseline visit (Visit 1) prior to Orkambi and the 24 week visit (Visit 3) after clinically prescribed Orkambi treatment has begun, and will last up to 2 days for the 12 week visit (Visit 2) after Orkambi treatment has begun.

Exclusion Criteria:

• On parenteral nutrition
• Use of any medications that inhibit or induce cytochrome P450 (CYP) 3A
• Liver function tests elevated above 3x the reference range for age and sex
• Lung disease considered severe based on clinical impression by home CF center.
• Other illness affecting growth or nutritional status
• Other contraindications described for Orkambi therapy

2 to 5 year old subjects with a diagnosis of cystic fibrosis and homozygous for F508del mutations and who are in a usual state of good health with a clinical decision to start orkambi treatment.