The glycosylphosphatidylinositol (GPI) anchor is a glycolipid found on many blood cells that serves to anchor proteins to the cell surface. This gene encodes a protein that is localized to the endoplasmic reticulum, and is involved in GPI anchor biosynthesis. As shown for the yeast homolog, which is a member of a family of dolichol-phosphate-mannose (Dol-P-Man)-dependent mannosyltransferases, this protein can also add a side-branching fourth mannose to GPI precursors during the assembly of GPI anchors. [provided by RefSeq, Jul 2008].