Endoplasmic reticulum mannosyl-oligosaccharide 1,2-alpha-mannosidase (EC 3.2.1.113) (ER alpha-1,2-mannosidase) (Mannosidase alpha class 1B member 1) (Man9GlcNAc2-specific-processing alpha-mannosidase); Involved in the maturation of Asn-linked oligosaccharides. Trim a single alpha-1,2-linked mannose residue from Man(9)GlcNAc(2) to produce Man(8)GlcNAc(2). The only product is the Man(8)GlcNAc(2) isomer B, the form lacking the middle-arm terminal alpha 1,2-mannose. It may be involved in glycoprotein quality control since it is important to target misfolded glycoproteins for degradation (865 aa)