GPC6 and esophageal squamous cell carcinoma: Glypican proteoglycan (GPC1 and GPC6), core membrane-anchored heparan sulfate proteoglycans, were upregulated in ESCC tissues compared with para-carcinoma tissues, whereas syndecan proteoglycans (SDC1, SDC2, and SDC4), transmembrane heparan sulfate proteoglycans, were downregulated (Fig. S5B).