Here we have established that B4GAT1, previously referred to as B3GNT1, is a xylopyranoside β1,4-glucuronyltransferase that appears to be the priming enzyme for the LARGE copolymerase for building the functional O-Man structure on α-dystroglycan that when defective causes CMD. The gene discussed is DAG1; the disease is congenital muscular dystrophy.