Required for protein O-linked mannosylation. Probably acts as a nucleotidyltransferase involved in synthesis of a nucleotide sugar. Required for dystroglycan O-mannosylation. Defects in ISPD are the cause of muscular dystrophy- dystroglycanopathy congenital with brain and eye anomalies type A7 (MDDGA7). MDDGA7 is an 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. Belongs to the IspD family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.