In yeasts and other fungi, O-mannosyl glycans constitute a major protein modification that is essential for cell viability. For several decades, protein O-mannosylation was considered a yeast-specific modification. Thus, it was especially interesting when it became evident that O-mannosyl glycans in mammals are not as rare as previously thought. O-mannosyl glycans are abundant in the mammalian brain and are also an abundant modification of alpha-dystroglycan, a component of the dystrophin-glycoprotein complex. Recently, mutations in genes that are or might be involved in the glycosylation of alpha-dystroglycan have been identified. Their association with neuromuscular diseases has focused the attention of different research areas on protein O-mannosylation.