In orthotopic xenograft mouse models of gastric and pancreatic cancers, platelet-derived serotonin enhances PD-L1 expression on cancer cells via transglutaminase 2 (TGM2)-mediated serotonylation of histones, consequently inhibiting the immune system’s anti-tumor efficacy and facilitating tumor proliferation (74). The gene discussed is TGM2; the disease is cancer.