PDCD1 and metastatic melanoma: Immune checkpoint inhibitors (ICIs) such as anti-PD1 (programmed cell death 1) and anti-CTLA4 (cytotoxic T lymphocyte antigen 4) antibodies have revolutionized the therapeutic landscape of metastatic melanoma and are approved as first-line therapies in the advanced as well as the adjuvant setting [1,2,3,4].