In addition to their expression on the plasma membrane of tumor cells, a variety of Tn/STn-carrying glycoproteins, such as mucins, CA 15-3 (MUC1), CA 125 (MUC16), and CA 72-4 (MUC1), are secreted in circulation in patients with gynecologic cancer [43,44,45,46,47,48,49], digestive cancer [50], gastric cancer [51,52], pancreatic cancer [53], and endometrial cancer [54], and the titers of those serum glycoproteins correlate with cancer malignancies [55,56]. Here, MUC1 is linked to gastric cancer.