Among the many checkpoint receptors of immune cells, cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), PD-1, T-cell immunoreceptor with immunoglobulin and ITIM domain (TIGIT), T-cell immunoglobulin-3 (TIM-3), and lymphocyte activation gene 3 (LAG-3) are those most commonly targeted in cancer immunotherapy. The gene discussed is PDCD1; the disease is cancer.