PDCD1 and melanoma: Anti-PD-1/PD-L1 immunotherapies have led to tremendous success in treating certain cancers, such as melanoma, non-small cell lung cancer (NSCLC), mCRC, renal cell carcinoma, bladder cancer, head and neck squamous cell carcinoma, classical Hodgkin’s lymphoma, and Merkel cell carcinoma [1–6, 57–63].