Nivolumab, a programmed cell death protein-1 inhibitor (anti-PD-1), and ipilimumab, a cytotoxic lymphocyte antigen-4 inhibitor (anti-CTLA-4), are approved for use as single agents and as a dual checkpoint inhibitor combination therapy for patients with unresectable or metastatic melanoma 2, 3. This evidence concerns the gene PDCD1 and metastatic melanoma.