PDCD1 and melanoma: In 2011, ipilimumab (an anti-CTLA-4 antibody) was approved for the treatment of malignant melanoma in the US, followed by the approval and clinical introduction of two anti-PD-1 antibodies (nivolumab and pembrolizumab) and three anti-PD-L1 antibodies (atezolizumab, avelumab, and durvalumab) for the treatment of several types of cancer.