... is a polar, covalent, naturally occurring, group II amino acid. Its oxidized form, cystine, was first isolated from urine (ew) by William Hyde Wollaston in 1812.  It can be synthesized from methionine and serine. Now you may be wondering, why is this chemical part of my story? (Refer to the story page.) What does cysteine do? (Refer to the uses page.)