Selenocysteine (Sec) is a nonstandard amino acid present in a minor fraction of proteins, called selenoproteins. Sec is an analog of cysteine (Cys) with selenium replacing sulfur. Sec is inserted during translation, in correspondence to specific UGA codons (normally stop signals) recoded in the presence of peculiar RNA structures. Sec is generally used in key sites of oxidoreductase enzymes. Its biosynthesis and insertion require a set of trans-factors collectively called Sec machinery, including a Sec-tRNA and a Sec-specific elongation factor.