PDCD1 and metastatic melanoma: Immune checkpoint inhibition with monoclonal antibodies targeting the programmed cell death-1 (PD-1) protein or its ligand, PD-L1, has produced significant clinical results in the treatment of several cancers, most notably metastatic melanoma [9, 10] and non-small cell lung cancer (NSCLC) [11, 12], and has shown promise in preclinical studies for the treatment of GBM [13, 14].