Muscle-eye-brain disease (MEB), an autosomal recessive disorder, is characterized by congenital muscular dystrophy, brain malformation, and ocular abnormalities. Previously, we found that MEB is caused by mutations in the gene encoding the protein O-linked mannose beta1,2-N-acetylglucosaminyltransferase 1 (POMGnT1), which is responsible for the formation of the GlcNAcbeta1-2Man linkage of O-mannosyl glycan. Although 13 mutations have been identified in patients with MEB, only the protein with the most frequently observed splicing site mutation has been studied. This protein was found to have no activity. Here, we expressed the remaining mutant POMGnT1s and found that none of them had any activity. These results clearly demonstrate that MEB is inherited as a loss-of-function of POMGnT1.