CFTR and cystic fibrosis: Thus, a short tract of five thymidines (5T) in intron 8 of the CFTR gene, found in ~10 % of individuals from the general population, can give rise to either congenital absence of the vas deferens (CAVD), non-classical cystic fibrosis or a normal phenotype when found in trans to a severe CFTR mutation (Kiesewetter et al. 1993; Cuppens et al. 1998).