CBS and hyperhomocysteinemia: This role is supported by the biochemical data (since homocysteine is a main substrate of the enzyme), animal data (CBS-deficient animals develop hyperhomocysteinemia [98,99,100,101,102,103,104,105,106,107,108]) and clinical data (inactivating CBS mutations result in classical homocystinuria, a rare inborn error of sulfur amino acid metabolism; see below).