Methanocaldococcus jannaschii (formerly Methanococcus jannaschii) is a thermophilic methanogenic archaean in the class Methanococci. It was the first archaeon to have its complete genome sequenced. The sequencing identified many genes unique to the archaea. Many of the synthesis pathways for methanogenic cofactors were worked out biochemically in this organism, as were several other archaeal-specific metabolic pathways.