HIF1A and neoplasm: Under hypoxia, accumulation of hypoxia-inducable factor-1 (HIF-1) activates hypoxia- adapted genes, which induces the expression of the immunosuppressive molecule PD-L1 in tumor cells, thereby increasing the resistance of tumor cells to cytotoxic lymphocyte (CTL)-mediated lysis and inducing CTL apoptosis (118–123).