noun. An amino acid, C₃H₇O₂NS, derived from cystine and found in most proteins.

noun. A sulphur-containing nonessential amino acid C3H7NO2S found in most animal proteins; it readily oxidizes to cystine.

noun. an amino acid containing sulfur that is found in most proteins; oxidizes on exposure to air to form cystine

"Bacteria start self-cleaving in response to a molecule inside cells called InsP6, which in turn activates a process called cysteine protease."