␣-Mannosidase IIx (MX) is an enzyme closely related to ␣-mannosidase II (MII), a key enzyme in N-glycan biosynthesis that catalyzes the first step in conversion of hybridto complex-type N-glycans in Golgi apparatus. Recently we generated MII/MX double knock-out mice and found that double nulls completely lack the complex-type Nglycans (Akama, T. O., Nakagawa, H., Wong, N. K., SuttonSmith, M., Dell, A., Morris, H. R., Nakayama, J., Nishimura, S. N-Glycosylation of proteins is one of the most ubiquitous post-translational modifications in eukaryotes and is thought to play an important role in development, differentiation, tumor metastasis, immunity, intracellular transport, cell adhesion, and aging (1). Insufficiency of human N-glycan biosynthesis leads to diseases such as congenital disorders of glycosylation (2) and hereditary erythroblastic multinuclearity with positive acidifies serum lysis test (3). N-Glycan biosynthesis involves five steps: (i) formation of a dolichol-linked oligosaccharide intermediate in the endoplasmic reticulum (ER), 1 (ii) en bloc transfer of oligosaccharide Glc3Man9GlcNAc2 from dolichol to Asn residue in the consensus sequence of Asn-X-Ser/ Thr on the nascent polypeptide chain by oligosaccharyltransFrom the ‡Laboratory of Advanced Chemical Biology, Graduate