Selenium is an important nutrient and its levels inside must be regulated since the lack of selenium suppresses expression of various enzymes, which leads to cell abnormality and diseases but high concentrations are toxic to the cell because it adversely affects numerous cell metabolic pathways. Structural domains comprising this superfamily share the structure of the subunits that comprise the Seleniun-binding protein (SeBP) which meadiates selienium transport in the archaeal Methanococcales. SeBP is a symmetric homopentamer that sequesters selenium during transport and therefore regulates the level of free selenium in the cell, and delivers it specifically to the selenophosphate synthase enzyme PMID:18650445.