Transfers mannosyl residues to the hydroxyl group of serine or threonine residues. Coexpression of both POMT1 and POMT2 is necessary for enzyme activity, expression of either POMT1 or POMT2 alone is insufficient. Defects in POMT2 are the cause of muscular dystrophy- dystroglycanopathy congenital with brain and eye anomalies type A2 (MDDGA2); also called Walker-Warburg syndrome or muscle-eye-brain disease POMT2-related. MDDGA2 is a autosomal recessive disorder characterized by congenital muscular dystrophy associated with cobblestone lissencephaly and other brain anomalies, eye malformations, profound mental retardation, and death usually in the first years of life. Included diseases are the more severe Walker-Warburg syndrome and the slightly less severe muscle-eye-brain disease. Defects in POMT2 are the cause of muscular dystrophy- dystroglycanopathy congenital with mental retardation type B2 (MDDGB2); also called muscular dystrophy congenital POMT2-related. MDDGB2 is an autosomal recessive disorder characterized by congenital muscular dystrophy associated with mental retardation and mild structural brain abnormalities. Defects in POMT2 are the cause of muscular dystrophy- dystroglycanopathy limb-girdle type C2 (MDDGC2); also called limb-girdle muscular dystrophy type 2N (LGMD2N) or muscular dystrophy-dystroglycanopathy limb-girdle POMT2-related (MDGD2C). MDDGC2 is an autosomal recessive muscular dystrophy with onset after ambulation is achieved. MDDGC2 is characterized by increased serum creatine kinase and mild muscle weakness. Muscle biopsy shows dystrophic changes, inflammatory changes, and severely decreased alpha-dystroglycan. Cognition is normal. Belongs to the glycosyltransferase 39 family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.