PDCD1 and metastatic melanoma: Immunotherapy, targeting specific immune checkpoints including cytotoxic T-lymphocyte antigen 4 (CTLA-4), programmed cell death 1 (PD1), and programmed cell death ligand 1 (PD-L1) has revolutionized the treatment of both locally advanced and metastatic melanoma and non-melanoma skin cancer (1, 2).