Immune checkpoint inhibitors (ICIs) including monoclonal antibodies (mAbs) against cytotoxic T-lymphocyte–associated antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed cell death ligand 1 (PD-L1) have significantly improved cancer treatment and achieved unprecedented efficacy in some types of cancer (Sasidharan Nair and Elkord, 2018; Togashi et al., 2019). The gene discussed is PDCD1; the disease is cancer.