Recently a number of alpha-amino acid transport proteins and corresponding cDNA clones have been isolated and categorized into gene families. The "CAT family" contains two members that mediate high-affinity Na(+)-independent transport of cationic amino acids in many tissues, and a third member that encodes a liver-specific low-affinity activity. The "glutamate transporter family" contains at least four members that mediate Na(+)-dependent glutamate/aspartate uptake and two members that are selective for neutral amino acids. The glutamate transporters are expressed at high levels in both glia and neurons of the central nervous system. The Na+/Cl(-)-dependent proline transporter (PROT) belongs to a large superfamily of neurotransmitter transporters and is expressed in regions of the brain that contain glutamanergic neurons. All four glycine transporters of the "GLYT family" also belong to the neurotransmitter superfamily and exhibit the greatest expression in the central nervous system. The "rBAT/4F2hc family" of proteins induce both neutral and cationic amino acid uptake when expressed in Xenopus oocytes. Cystinuria is linked to specific mutations in the rBAT sequence.