The 22nd known amino acid discovered by Joseph A. Krzycki and Michael K. Chan of the Ohio State University in Columbus and their colleagues in 2002.
As part of the methyl group it was first believed to be a stop codon, but after deep investigation on the production of methane by the Methanosarcina barkeri they found that, under somes unusuals circumstances, instead of stopping the mRNA translation, this codon translate into a totally new amino acid.
For those of you who want to play god, here is the new LEGO
block in your set. The three letters of the codon are UAG.