CF (cystic fibrosis) is a recessive genetic disease caused by mutations of the CFTR (cystic fibrosis transmembrane conductance regulator), a cAMP-activated anion channel, exhibiting a multitude of clinical manifestations including lung inflammation/infection, pancreatic insufficiency/diabetes, intestinal obstruction and infertility in both sexes. While mutation DF508 is found in 70% of CF patients, large variation in disease phenotypes and severity is observed among the patients. This review discusses current theories accounting for the disease variations and puts forth an epigenetic hypothesis involving microRNAs.