PDCD1 and cancer: PD-1 and CTLA-4 are co-inhibitory receptors expressed on the surface of T-lymphocytes, where they bind to their ligand (PDL-1 and CD80/CD86 ligands; respectively) expressed on the surface of regulatory immune cells and some types of cancers (Beckers et al. 2016; Cimino-Mathews et al. 2016).