PDCD1 and melanoma: Since Pembrolizumab, as a humanized monoclonal anti-PD-1 antibody, was approved by FDA for treating patients with advanced melanoma and non-small-cell lung cancer (NSCLC) in 2014 [21], at least six more anti-PD-1 or anti-PD-L1 antibodies have been approved then put into clinical application [17].