Cystic fibrosis. Cystic fibrosis is caused by a mutation that affects the function of a chloride ion channel. (a) The CFTR gene is on chromosome 7. It encodes a channel protein that transports chloride ions. The most common CFTR mutation leading to cystic fibrosis is a deletion of three nucleotides that results in the omission of phenylalanine (Phe) at position 508. The isoleucine (Ile) at position 507 remains the same, because both ATC and ATT code for an Ile residue. The omission of Phe508 prevents proper protein folding. (b) The chloride ion channel consists of five domains: two domains that form the channel across the cytoplasmic membrane (red), two domains that bind and use ATP as an energy source (ATP1 and ATP2), and a regulatory domain. Phe508 is in the ATP1 domain.