PDCD1 and melanoma: The clinical development of inhibitors against programmed cell death 1 (PD-1) and its ligand (PD-L1) as anticancer agents has broadened since the approval of pembrolizumab for the treatment of advanced melanoma in September 20145,6, and currently, immune checkpoint inhibitors targeting the PD-1/PD-L1 axis are approved for the treatment of several other malignancies7.