The enzyme oligosaccharyltransferase (dolichyl-diphosphooligosaccharide–protein glycosyltransferase; EC 2.4.1.119) (DDOST) catalyzes the transfer of a high-mannose oligosaccharide (GlcNac 2 Man 9 Glc 3 ) from a dolichol-linked oligosaccharide donor (dolichol-P-GlcNac 2 Man 9 Glc 3 ) onto the asparagine acceptor site within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains across the membrane of the endoplasmic reticulum. We isolated mouse and human DDOST cDNAs from retinoic acid-treated mouse P19 EC cells and human NT-2 cells, respectively. DDOST mRNA is expressed intensely in heart and pancreas, but at lower levels in brain. Here we show that the human DDOST 48-kDa subunit gene (HGMW-approved symbol DDOST) is organized into 11 exons expanding about 9 kb. This DDOST subunit gene is localized on chromosome 1p36.1 by fluorescencein situhybridization analysis.