These sites (modifier loci) are often identified by genome‐wide‐association studies (GWAS).1 However, defining the mechanisms whereby modifier loci contribute to phenotype is often challenging.2 One example is cystic fibrosis (CF) a life‐shortening, recessive genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The gene discussed is CFTR; the disease is cystic fibrosis.