Selenocysteine (abbreviated as Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid. Selenocysteine exists naturally in three domains of life, but not in every lineage, as a building block of selenoproteins. Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group. Selenocysteine was discovered by biochemist Thressa Stadtman at the National Institutes of Health.