S-tert-Butyl-D-cysteine hydrochloride is a protected form of D-Cysteine (C995020). D-Cysteine is a strong inhibitor of Escherichia coli growth and also functions to provide inorganic sulfates for the sulfation of xenobiotics. D-Cysteine is a non-physiological isomer of L-Cysteine (C995000), and is not involved in protein or glutathione synthesis.