Participates in O-mannosyl glycosylation. May be responsible for the synthesis of the GlcNAc(beta1-2)Man(alpha1-)O- Ser/Thr moiety on alpha-dystroglycan and other O-mannosylated proteins. Is specific for alpha linked terminal mannose and does not have MGAT3, MGAT4, MGAT5, MGAT7 or MGAT8 activity. Defects in POMGNT1 are the cause of muscular dystrophy- dystroglycanopathy congenital with brain and eye anomalies type A3 (MDDGA3). MDDGA3 is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities, cobblestone lissencephaly and cerebellar hypoplasia. MDDGA3 patients present severe congenital myopia, congenital glaucoma, pallor of the optic disks, retinal hypoplasia, mental retardation, hydrocephalus, abnormal electroencephalograms, generalized muscle weakness and myoclonic jerks. Defects in POMGNT1 are the cause of muscular dystrophy- dystroglycanopathy congenital with mental retardation type B3 (MDDGB3); also called muscular dystrophy congenital POMGNT1-related. MDDGB3 is an autosomal recessive disorder characterized by congenital muscular dystrophy associated with mental retardation and mild structural brain abnormalities. Clinical features include mental retardation, white matter changes, cerebellar cysts, pontine hypoplasia, myopia, optic atrophy, decreased alpha-dystroglycan on muscle biopsy and increased serum creatine kinase. Defects in POMGNT1 are the cause of muscular dystrophy- dystroglycanopathy limb-girdle type C3 (MDDGC3); also called muscular dystrophy-dystroglycanopathy limb-girdle POMGNT1- related. MDDGC3 is a rare form of limb-girdle muscular dystrophy with normal cognition. Muscle biopsy shows dystrophic changes with variable staining for glycosylated alpha-dystroglycan. Belongs to the glycosyltransferase 13 family. Note: This description may include information from UniProtKB.