Several non-proteinogenic amino acids are toxic due to their ability to mimic certain properties of proteinogenic amino acids, such as thialysine. Some non-proteinogenic amino acids are neurotoxic by mimicking amino acids used as neurotransmitters (i.e. not for protein biosynthesis), e.g. Quisqualic acid, canavanine or azetidine-2-carboxylic acid.[21] Cephalosporin C has an α-aminoadipic acid (homoglutamate) backbone that is amidated with a cephalosporin moiety.[22] Penicillamine is a therapeutic amino acid, whose mode of action is unknown.