Walker-Warburg syndrome (WWS) is an autosomal recessive developmental disorder characterized by congenital muscular dystrophy, brain malformation, and structural eye abnormalities. WWS is due to defects in protein O-mannosyltransferase 1 (POMT1), which catalyzes the transfer of mannose to protein to form O-mannosyl glycans. POMT1 has been shown to require co-expression of another homologue, POMT2, to have activity. In the present study, mutations in POMT1 genes observed in patients with WWS were duplicated by site-directed mutagenesis. The mutant genes were co-expressed with POMT2 in Sf9 cells and assayed for protein O-mannosyltransferase activity. Expression of all mutant proteins was confirmed by Western blot, but the recombinant proteins did not show any protein O-mannosyltransferase activity. The results indicate that mutations in the POMT1 gene result in a defect of protein O-mannosylation in WWS patients. This may cause failure of binding between alpha-dystroglycan and laminin or other molecules in the extracellular matrix and interrupt normal muscular function and migration of neurons in developing brain.