1 and 7). PMTs catalyze the transfer of mannose from dolichol-phosphate mannose (Dol-P-Man) to the hydroxyl group of serine or threonine residues of proteins in the lumen of the ER. These enzymes are essential for the viability of yeasts, filamentous fungi, and animals (5, 8, –, 12). Numerous mutations in the human PMTs (POMT1, POMT2) have been identified that cause various forms of α-dystroglycanopathies, with Walker-Warburg syndrome (WWS) being the most severe form of these disorders (13, –, 16). This evidence concerns the gene POMT1 and muscular dystrophy-dystroglycanopathy, type A.