CD274 and neoplasm: For example, HIF reduces the expression of major histocompatibility complex I (MHC I) while increasing the expression of PD‐L1, thereby reducing the tumor cell killing ability of cytotoxic T lymphocytes (CTLs),418, 419 while hyperoxia (60% O2) upregulates MHC expression, confirming that MHC antigen presentation regulation is oxygen‐dependent.420