This gene encodes a type I transmembrane protein in the endoplasmic reticulum (ER). The protein is an essential component of glycosylphosphatidylinositol-mannosyltransferase I, which transfers the first of the four mannoses in the GPI-anchor precursors during GPI-anchor biosynthesis. Studies in rat indicate that the protein is translated from a non-AUG translation initiation site. Multiple transcript variants encoding different isoforms have been found for this gene. [provided by RefSeq, Jul 2008]