Statistical analysis. The sample size estimate is based on the absolute change from baseline of the cumulative chloride secretory ion current response to forskolin/IBMX and carbachol in ICM as outcome measure of CFTR function. Assuming a nominal type I error of 0.05 and a power of 0.8, 125 or 33 subjects are needed to demonstrate a treatment effect of 5% or 10%, respectively. Thus even modest changes in the basis defect can be demonstrated in a recruited cohort of 125 subjects and incomplete data sets in up to half of all subjects.