Our results further define the functional O-mannosylated glycan structure and indicate that B4GAT1 is involved in the initiation of the LARGE-dependent repeating disaccharide that is necessary for extracellular matrix protein binding to O-mannosylated α-dystroglycan that is lacking in secondary dystroglycanopathies. The gene discussed is DAG1; the disease is neuromuscular disease caused by qualitative or quantitative defects of alpha-dystroglycan.