Tumor cells exploit immune regulatory mechanisms to evade the immune system by activating two negative regulatory mechanisms for T cell response (proliferation, pro-survival, cytotoxicity): the programmed death 1 (PD1)/PD1 ligand (PD-L) and the cytotoxic T-lymphocyte associated antigen 4 (CTLA4)/B7.146. The gene discussed is CTLA4; the disease is neoplasm.