We have identified three different point mutations in the coding region of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Each mutation segregates with the disease in two- or three-generation pedigrees and is not found on the normal chromosome of any documented cystic fibrosis carrier. One of the mutations is found in two independent families that contain at least one individual with a mild course of disease. All of these alterations replace charged amino acids with less polar residues and are found in the putative transmembrane sections of the molecule. The mutated amino acids are found to be conserved in both rodents and amphibians and lie in a region of CFTR that is believed to form a channel in the membrane. Although these alterations are rare, they provide important clues to functionally important regions of the molecule.