Immune checkpoint inhibitors (ICIs) targeting programmed cell death-1 (PD-1), programmed cell death-ligand 1 (PD-L1) or cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) have become widely studied and used immunotherapies because of their impressive clinical results in some cancer types, such as melanoma. The gene discussed is PDCD1; the disease is cancer.