CTLA4 and cancer: Cytotoxic T lymphocyte-associated protein 4 (CTLA4) and programmed death 1 (PD1) are the two best-studied immune checkpoint molecules, and currently, they can both be targeted by humanized antibodies that have been approved by the U.S. Food and Drug Administration (FDA) for clinical use (Figure 2); these antibodies have exhibited unprecedented efficacy in several cancer indications.