As success with checkpoint inhibitors (anti-PD-1, -PD-L1, -CTLA-4) and adoptive cell transfer (ACT, including chimeric antigen receptor (CAR) and tumor-infiltrating lymphocyte (TIL)) becomes more prevalent, the interest in the regulation of the immune system in the tumor microenvironment (TME) has grown. The gene discussed is PDCD1; the disease is neoplasm.