Cystic fibrosis (CF), the most frequent lethal autosomal recessive disease in Caucasians, is caused by more than 1,000 different mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene listed in the cystic fibrosis mutation database located at https://www.genet.sickkids.on.ca/cftr/. As a consequence of the wide spectrum of mutations that are present, the clinical presentation of CF varies widely from monosymptomatic disease to multiorgan involvement (568). Some CFTR mutations confer residual CFTR function in rectal epithelia, which results in a milder clinical phenotype (569).