CTLA4 and cancer: During the 1990s, the immune checkpoint key proteins, cytotoxic T lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein (PD-1), and programmed death-ligand 1 (PD-L1), revolutionized cancer therapy (Korman et al., 2006; Okazaki and Honjo, 2007).