This gene encodes an enzyme belonging to the glycosyl hydrolase 47 family. This enzyme functions in N-glycan biosynthesis, and is a class I alpha-1,2-mannosidase that specifically converts Man9GlcNAc to Man8GlcNAc isomer B. It is required for N-glycan trimming to Man5-6GlcNAc2 in the endoplasmic-reticulum-associated degradation pathway. Mutations in this gene cause autosomal-recessive intellectual disability. Alternative splicing results in multiple transcript variants. A related pseudogene has been identified on chromosome 11. MAN1B1 (Mannosidase Alpha Class 1B Member 1) is a Protein Coding gene. Diseases associated with MAN1B1 include Mental Retardation, Autosomal Recessive 15 and Man1b1-Cdg. Among its related pathways are Calnexin/calreticulin cycle and Mechanisms of CFTR activation by S-nitrosoglutathione (normal and CF). GO annotations related to this gene include calcium ion binding and mannosyl-oligosaccharide 1,2-alpha-mannosidase activity. An important paralog of this gene is MAN1A2.