Proteinogenic amino acids are amino acids that are precursors to proteins, and are incorporated into proteins during translation. Throughout known life, there are 23 proteinogenic amino acids, 20 in the standard genetic code and an additional 3 that can be incorporated by special translation mechanisms. Both eukaryotes and prokaryotes can incorporate selenocysteine into their proteins via a nucleotide sequence known as a SECIS element, which directs the cell to translate a nearby UGA codon as selenocysteine (UGA is normally a stop codon). In some methanogenic prokaryotes, the UAG codon (normally a stop codon) can also be translated to pyrrolysine. In bacteria, the AUG initiation codon is translated to N-formylmethionine when it is actually used to initiate translation and translated normally (to methionine) at other times. In eukaryotes, there are only 21 proteinogenic amino acids, the 20 of the standard genetic code, plus selenocysteine. Humans can synthesize 12 of these from each other or from other molecules of intermediary metabolism. The other nine must be consumed (usually as their protein derivatives), and so they are called essential amino acids. The essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (i.e. H I L K M F T W V).