CD274 and neoplasm: Building on this, a tumor classification system [119] has been proposed where type I tumors are PD-L1-/TIL- (immune ignorance), type II tumors are PD-L1+/TIL+ (acquired immune resistance), type III tumors are PD-L1-/TIL+ (immune tolerance by other suppressor factors), and type IV tumors are PD-L1+/TIL- (constitutive expression of PD-L1) [120].