PDCD1 and melanoma: The immunological components of the tissue microenvironment are currently considered fundamental to regulate tumoral progression, so that the programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) immune check point is targeted by specific inhibitors in a number of different malignancies including melanoma, non-small cell lung cancer, renal cell carcinoma, head and neck cancer and bladder cancer.