PDCD1 and metastatic melanoma: The approval of the anti–CTLA-4 antibody ipilimumab for metastatic melanoma in 2011 — a substantial milestone — was followed in 2014 with anti–PD-1 antibodies nivolumab and pembrolizumab to treat advanced or metastatic melanoma and as frontline therapy for high-risk patients in 2016 (Figure 1).