PDCD1 and melanoma: ICIs work against the immune checkpoint modulators such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4; ipilimumab), programmed cell death protein 1 (PD-1; nivolumab, pembrolizumab), and its ligand (PD-L1; atezolizumab, avelumab, durvalumab), and are used in the treatment of various cancers, including lung cancer, kidney cancer, bladder cancer, prostate cancer, lymphomas, and malignant melanomas [1].