N-Acetyl-S-[2-hydroxy-2-(3-pyridinyl)ethyl]-L-cysteine-D3 is deuterium labeled N-Acetyl-S-[2-hydroxy-2-(3-pyridinyl)ethyl]-L-cysteine (A179085), a modified L-cysteine that can be incorporated into synthetic peptide/protein and used as a potential biochemical probe to investigate biological systems.