CFTR and cystic fibrosis: In particular, debate surrounds whether muscle size (affected by nutritional compromise, attenuated pubertal growth, and catabolism during pulmonary exacerbations) or its intrinsic function [via expression of the CF Transmembrane Conductance Regulator (CFTR) protein within the sarcoplasm (6)] contribute towards skeletal muscle metabolism and reduced V̇O2max in people with CF.