PDCD1 and metastatic melanoma: Thus far, four antibodies have been licensed: (1) ipilimumab, an antibody against CTLA-4, has been licensed for unresectable or metastatic melanoma; (2) two mAbs against PD-1, pembrolizumab and nivolumab, have been approved for unresectable metastatic melanoma and advanced metastatic non-small-cell lung carcinoma.