Classes I–III tend to abolish CFTR expression and/or function (severe mutations) whereas classes IV–VI produce CFTR variants with residual expression and/or function (mild mutations).2 Although the genotype–phenotype correlation is strong for pancreatic disease (severe mutations lead to pancreatic insufficiency and patients require enzyme supplements to digest food, whereas patients with mild mutations remain pancreatically sufficient), in the lung the environment, socioeconomic factors, and other modifier genes significantly contribute to disease severity. Here, CFTR is linked to exocrine pancreatic insufficiency.