CTLA4 and cancer: From the six major immune checkpoints for cancer immunotherapy, namely cytotoxic T-lymphocyte associated protein 4 (CTLA-4), programmed cell death 1 (PDCD1), cluster of differentiation 274 (CD274), inducible T-cell costimulator (ICOS), lymphocyte-activation gene 3 (LAG3), and CD40, ipilimumab targeted against CTLA-4 was the first monoclonal antibody approved by the Food and Drug Administration (FDA) in 2011.