PDCD1 and melanoma: In patients with advanced melanoma, treatment with immune checkpoint inhibitor (ICI) monoclonal antibodies targeting programmed cell death protein 1 (PD-1) (e.g., nivolumab) either alone or in combination with anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) (e.g., ipilimumab) improves survival outcomes and are now standard-of-care treatment options (3–5).