This gene encodes a GDP-Man:Man3GlcNAc2-PP-dolichol-alpha1,2-mannosyltransferase which is localized to the cytosolic side of the endoplasmic reticulum (ER) and catalyzes the transfer of the fourth and fiftHuman, Mouseannose residue from GDP-mannose (GDP-Man) to Man3GlcNAc2-PP-dolichol and Man4GlcNAc2-PP-dolichol resulting in the production of Man5GlcNAc2-PP-dolichol. Mutations in this gene are associated with congenital disorder of glycosylation type Ip (CDGIP).