Deficiencies in the protein O-mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGNT1), a glycosyltransferase involved in the elongation of classic O-mannosyl glycans, are mainly associated with muscle-eye-brain disease (MEB), a genetically heterogeneous congenital muscular dystrophy with brain and eye anomalies. Although occasional aberrant cell-cell adhesion has been observed in response to changes in POMGNT1 levels, direct molecular mechanisms are largely unknown. POMGNT1 knock-out HEK293T cells and fibroblasts derived from MEB patients were used to get deeper insight in POMGNT1 deficiency by the combination of biochemical, cell and molecular biological techniques with proteomics and glycan-profile.