PDCD1 and metastatic melanoma: Currently, checkpoint inhibitor therapy (CIT) applying the immune checkpoint-specific antibodies against the cytotoxic T-lymphocyte-associated protein-4 (CTLA-4; ipilimumab) and programmed cell death protein-1 (PD-1; nivolumab and pembrolizumab) are approved as first-line treatments for patients with metastatic melanoma 2.