many proteins. In this case, the symbol Cyx is sometimes used. The deprotonated form can generally be described by the symbol Cym as well.

When used as a food additive, cysteine has the E number E920.

Cysteine is encoded by the codons UGU and UGC.

Structure

Like other amino acids (not as a residue of a protein), cysteine exists as a zwitterion.