Importantly, the ligands of CTLA-4 B7-1 (CD80) and B7-2 (CD86) are expressed on antigen presenting cells (APC) but not on cancer cells, whereas the ligands of PD-1, PD-L1 and PD-L2, are expressed on APC and on cancer cells [12, 13]. This evidence concerns the gene PDCD1 and cancer.