PDCD1 and metastatic melanoma: Checkpoint inhibitors of programmed cell death 1 (PD-1) and its ligands (PD-L1 and PD-L2) and cytotoxic lymphocyte antigen protein 4 (CTLA-4) are an emerging class of agents currently licensed in metastatic melanoma, NSCLC and Hodgkin’s lymphoma [50,51,52].