Genetic engineering approach transforms the entire GlaNAc-type O-glycoproteome of a cell into homogeneous simple truncated O-glycans with GalNAc (Tn antigen) and/or NeuAcα2-6GalNAc (STn antigen) structures. This is achieved by zinc finger nuclease (ZFN) mediated knockout of either the core-extending glycosyltransferase (T-synthase) or its essential chaperone (cosmc)