AbstractDefects in man in four steps of 4‐aminobutyric acid (GABA) metabolism may interefere with the function of this major inhibitory neurotransmitter. Glutamic acid decarboxylase, 4‐aminobutyric acid aminotransferase, succinic semialdehyde dehydrogenase, and homocarnosinase are closely identified with the brain, but two of these enzymes are expressed in cultured peripheral cells, which may permit novel approaches to the study of the metabolism and regulation of G