PDCD1 and melanoma: Since approved for clinical use, ICIs targeting humanized cytotoxic T lymphocyte antigen 4 (CTLA4), programmed cell death 1 (PD-1) or PD-1 ligand 1 (PD-L1) have been used to treat an ever-growing list of malignant tumors, such as melanoma, lung cancers, head and neck squamous cell cancer, bladder cancer (BLCA), and gastro-esophageal cancer (3).