CD274 and neoplasm: Based on PD-L1 expression and the presence of tumor-infiltrating lymphocytes (TIL), human tumors have been classified into four types [46]: type I, PD-L1+TIL+ (adaptive immune resistance, where PD-L1 expression is induced by TIL); type II, PD-L1−TIL− (immune ignorance); type III, PD-L1+TIL− (the intrinsic induction of PD-L1 expression); and type IV, PD-L1−TIL+ (immune tolerance, where factors other than PD-L1 expression are implicated in the evasion of the host’s immunity).