Mutations of the protein O-mannosyltransferase (POMT1) gene affect glycosylation of alpha-dystroglycan, leading to Walker-Warburg syndrome, a lethal disorder in early life with severe congenital muscular dystrophy, and brain and eye malformations. Recently, we described a novel form of recessive limb girdle muscular dystrophy with mild mental retardation, associated with an abnormal alpha-dystroglycan pattern in the muscle, suggesting a glycosylation defect. Here, we present evidence that this distinct phenotype results from a common mutation (A200P) in the POMT1 gene. Our findings further expand the phenotype of glycosylation disorders linked to POMT1 mutations. Furthermore, the A200P mutation is part of a conserved core haplotype, indicating an ancestral founder mutation.