Endoplasmic Reticulum Mannosyl-Oligosaccharide 1,2-α-Mannosidase (MAN1B1) belongs to the glycosyl hydrolase 47 family. MAB1B1 is a single-pass type II membrane protein and widely expressed in many tissues. MAB1B1 is involved in glycoprotein quality control targeting of misfolded glycoproteins for degradation. MAB1B1 can be inhibited by both 1-deoxymannojirimycin (dMNJ) and kifunensine. Defects in MAN1B1 are the cause of mental retardation autosomal recessive type 15 (MRT15). Mental retardation is characterized by significantly below average general intellectual functioning, it is also associated with impairments in adaptative behavior and manifested during the developmental period.