CD274 and cancer: The most broadly studied checkpoint proteins are engaged in the surface of both T cells and cancer cells, generally serving as receptor and ligands, including CTLA-4/CD80 or CD86, PD-1/PD-L1 or PD-L2, LAG3/MHC-II, TIM3/galectin-9, BTLA/HVEM, TIGIT/CD155 [79, 80].