Similar to CTLA-4, another coreceptor of the CD28 superfamily, PD-1, which is expressed on the surface of T cells, and its ligands PD-L1/PD-L2, which are expressed on the surface of tumor cells, together constitute another critical coinhibitory signaling pathway participating in regulating the adaptive immune response and are completely independent of B7-1: CTLA-4 (Fife and Bluestone, 2008; Gardner et al., 2014). Here, CTLA4 is linked to neoplasm.