Among them immune checkpoint inhibitors (CPIs), acting against PD-1 (programmed cell death protein 1), PD-L1 (programmed death ligand 1), and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), have exhibited antitumor activity in a variety of tumor types, including metastatic melanoma, lung cancer, and breast malignancies (5). The gene discussed is PDCD1; the disease is metastatic melanoma.