An amino acid isolated from Cholera vibrio: Blass, Macheboeuf, Helv. Chim. Acta 29, 1315 (1946). Occurrence in proteins or protein containing material: Windsor, J. Biol. Chem. 192, 595 (1951). Synthesis: Dieckmann, Ber. 38, 1656 (1905); T. P. Waalkes et al., J. Am. Chem. Soc. 72, 5760 (1952); A. I. Scott, T. J. Wilkinson, Synth. Commun. 10, 127 (1980).