Pyrrolysine (abbreviated as Pyl or O; encoded by the 'amber' stop codon UAG) is an ɑ-amino acid that is used in the biosynthesis of proteins in some methanogenic archaea and bacterium; it is not present in humans. It contains an α-amino group (which is in the protonated –⁺NH₃ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO^- form under biological conditions). Its pyrroline side-chain is similar to that of lysine in being basic and positively charged at at neutral pH.