Study of Lumacaftor in Combination With Ivacaftor in Subjects 6 Through 11 Years of Age With Cystic Fibrosis, Homozygous for the F508del-CFTR Mutation

Inclusion Criteria:

* Confirmed diagnosis of CF defined as: with 2 CF-causing mutations, chronic sinopulmonary disease or gastrointestinal/nutritional abnormalities
* Subjects who weigh ≥15 kg without shoes at Screening Visit
* Subjects who are homozygous for the F508del-CFTR mutation
* Subjects with percent predicted forced expiratory volume in 1 second (FEV1) of 70% to 105% (inclusive) (Part A) or ≥40% (Part B) at Screening Visit where the predicted values are adjusted for age, sex, and height using the Wang equation
* Subjects with stable CF disease and who are willing to remain on stable CF medication regimen
* Able to swallow tablets

Exclusion Criteria:

* History of any illness or condition that might confound the results of the study or pose an additional risk in administering study drug to the subject
* Acute respiratory infection, pulmonary exacerbation, or changes in therapy for pulmonary disease within 28 days before Day 1 of the study
* Abnormal liver function as defined in the protocol at Screening Visit
* Abnormal renal function as defined in the protocol at Screening Visit
* History of solid organ or hematological transplantation
* Ongoing participation in an investigational drug study or prior participation in an investigational drug study within 30 days prior of Screening Visit
* History or evidence of lens opacity or cataract at Screening Visit
* Colonization with organisms associated with a more rapid decline in pulmonary status at Screening Visit (Part A only)
* A standard 12-lead ECG demonstrating QTcF \>450 msec at Screening Visit