CFTR and cystic fibrosis: The vast majority of monogenic diseases are characterized by allelic heterogeneity (numerous mutations of a single gene, e.g., cystic fibrosis, caused by mutations of the CFTR gene, with about 2000 variants described in this gene) or nonallelic heterogeneity, which means that a given disease is caused by a mutation(-s) in one of several different genes (e.g., hearing loss).