3 ms·Cysteine is not an essential amino acid. Humans can synthesize it from methionine.by kens 1y agoCysteine is not an essential amino acid. Humans can synthesize it from methionine.bell-cot 1y agoWikipedia's disclaimers on that: "Cysteine can usually be synthesized by the human body under normal physiological conditions if a sufficient quantity of methionine is available."nkmnz 1y agoThese mice were GMO’d to be incapable of synthesising cysteine.