PDCD1 and melanoma: The novel development of the immune checkpoint inhibitor (ICI) is now approved in a variety of solid tumors, including melanoma, non-small cell lung cancer (NSCLC), and urothelial and microsatellite instability-high (MSI) cancer, represented by programmed cell death-1 (PD-1) and programmed cell death ligand-1 (PD-L1), which became a crucial therapeutic option to improve prognosis (1).