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 POMT1 are the cause of muscular dystrophy- dystroglycanopathy congenital with mental retardation type B1 (MDDGB1); also called muscular dystrophy congenital POMT1-related. MDDGB1 is an autosomal recessive disorder characterized by congenital muscular dystrophy associated with mental retardation and mild structural brain abnormalities. Defects in POMT1 are the cause of muscular dystrophy- dystroglycanopathy congenital with brain and eye anomalies type A1 (MDDGA1); also known as hydrocephalus-agyria-retinal dysplasia or HARD syndrome. MDDGA1 is an autosomal recessive disorder characterized by cobblestone lissencephaly, hydrocephalus, agyria, retinal displasia, with or without encephalocele. It is often associated with congenital muscular dystrophy and usually lethal within the first few months of life. Included diseases are the more severe Walker-Warburg syndrome and the slightly less severe muscle-eye-brain disease. Defects in POMT1 are the cause of muscular dystrophy- dystroglycanopathy limb-girdle type C1 (MDDGC1); also called autosomal recessive limb-girdle muscular dystrophy with mental retardation. MDDGC1 is a novel form of recessive limb girdle muscular dystrophy with mild mental retardation without any obvious structural brain abnormality, associated with an abnormal alpha-dystroglycan pattern in the muscle. MDDGC1 is a significantly milder allelic form of WWS. Belongs to the glycosyltransferase 39 family. 4 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.