PDCD1 and melanoma: Monoclonal antibody treatments of programmed cell death-1 (PD-1) and programmed death-ligand 1 (PD-L1) have shown promising therapeutic effects in many cancers, including melanoma, non-small cell lung cancer (NSCLC), renal cell carcinoma, head and neck squamous cell carcinoma, urothelial cancer, refractory Hodgkin lymphoma, breast cancer, and malignancies with microsatellite instability (Motzer et al., 2015; Ferris et al., 2016; Bellmunt et al., 2017; Lee et al., 2017; Hu J. R. et al., 2019).