A selenoprotein is any protein that includes a selenocysteine residue. Selenoproteins exist in all major forms of life, eukaryote, eubacteria and archaea. Among eukaryotes, selenoproteins appear to be common in animals, but rare or absent in other phyla (one has been identified in the green alga Chlamydomonas, but none in other plants or in fungi). Among eubacteria and archaea, selenoproteins are only present in some lineages, while they are completely absent in many other phylogenetic groups.