<prompt> Selenocysteine is synthesized fromAlamineCysteineArginineHistidine  <response>: Selenocysteine is the 21st amino acid. Biosynthesis of Selenocysteine is by replacing the oxygen by selenium in Serine which in turn formed from Cysteine.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 214 