This entry represents PylB, which is part of a three-gene cassette that directs the biosynthesis of pyrrolysine, the twenty-second amino acid, that is incorporated in some species at a UAG canonical stop codon [<cite idref="PUB00055034"/>]. PylB possesses signature residues suggesting it to be a radical SAM protein, which mediate radical-catalyzed reactions. It may be involved in pyrrolysine ring formation or methylation.