Impaired GMPPB function reduces the amount of GDP-mannose available for the O-mannosylation of α-dystroglycan (α-DG) and ultimately leads to disruptions of the link between α-DG and extracellular proteins, hence dystroglycanopathy. This evidence concerns the gene DAG1 and neuromuscular disease caused by qualitative or quantitative defects of alpha-dystroglycan.