PRG4 and cancer: Interestingly, a recent study capitalizing on the advances in glycoproteomics using innovative cancer cell genetic engineering [32, 33] has clearly identified proteoglycan 4 as one of a variety of O-glycoproteins with the truncated STn (NeuAcα2-6GalNAcα1-O-Ser/Thr) glycoform in the sera of gastric carcinoma patients [34].