PDCD1 and melanoma: Immune checkpoint inhibitors (ICIs), including anti-CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), anti-programmed cell death 1 (PD-1), and anti-programmed death ligand-1 (PD-L1), are currently used to treat various cancers, including melanoma, non-small cell lung cancer, renal cell carcinoma, and Hodgkin's lymphoma (Ribas and Wolchok, 2018; Xu et al., 2018; Khan and Gerber, 2019).