Statistical Analysis: Data will be presented as means ± standard deviation unless otherwise specified. Within-group comparison of exercise responses during constant-load exercise tests will be performed using paired t-tests with Bonferroni corrections where appropriate, comparing exercise responses during constant-load exercise tests performed before (visit 2) and after (visits 3 and 4) initiating full dose of Orkambi. Within-group comparison of exertional dyspnea will be based on Borg dyspnea ratings taken at iso-time, defined as the maximum time achieved on constant-load exercise tests performed on visits 2, 3, and 4. An additional analysis will include adjustment for potential covariates including age, sex, and baseline lung function (FVC and DLCO). Pearson correlation coefficients will be used to examine the association between measured variables (e.g. breathing patterns, ventilatory responses, operating lung volumes, pulmonary function variables, etc.) with changes in Borg ratings and endurance time. Reasons for stopping exercise and qualitative descriptors of dyspnea will be analyzed as frequency statistics and compared between constant -load exercise tests performed on visits 2, 3, and 4 using the McNemar's test at iso-time. A P-value less than 0.05 will be regarded as statistically significant. Statistical analysis of the data will be performed using Stata v11.2 (StataCorp, Texas, USA).