1 and 2). In humans, a heterogeneous group of congenital muscular dystrophies, collectively referred to as secondary α-dystroglycanopathies, is connected to the reduced O-mannosylation of the cell surface-associated basement membrane receptor α-dystroglycan (αDG) (reviewed in Ref. 3). Furthermore, O-mannosyl glycans are also commonly present among members of the cadherin and plexin families and influence cadherin-mediated cell-cell adhesion (4, –, 6). This evidence concerns the gene CDH17 and muscular dystrophy.