PDCD1 and melanoma: Recent advances in immune checkpoint blockade (ICB) therapy, such as targeting programmed cell death protein 1 (PD-1)/PD-ligand 1 (PD-L1) and/or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), illustrate the power of enhancing the patient’s endogenous anti-melanoma immune responses [4, 8–10].