It has been observed that in some tumors, cytotoxic T cells are repressed and cannot respond against the tumor through the interaction between T cell receptors and co-receptors [PD-1 and its corresponding ligands PD-L1/PD-L2 or B7.H1/B7.DC, and CD152/CTLA-4 and its ligands CD80 (B7-1) and CD86 (B7-2)] present in the cancer cell. This evidence concerns the gene CTLA4 and cancer.