CFTR and cystic fibrosis: To date, more than 2000 mutations of the cftr gene have been described (Cystic Fibrosis Mutation Database; www.genet.sickkids.on.ca), although the most frequent mutation causing CF disease is the deletion of three base pairs in both copies of the gene that causes the loss of phenylalanine residue at position 508 of the CFTR protein (F508del) [49].