CTLA4 and cancer: Therefore, immune checkpoint inhibitors (ICIs), including cytotoxic T lymphocyte antigen-4 (CTLA-4), programmed cell death protein 1 (PD-1)/programmed cell death-ligand 1 (PD-L1) axis, indoleamine 2,3-dioxygenase (IDO), etc., are now the frontline of immunotherapy for a variety of cancers 2.