PDCD1 and metastatic melanoma: Current research shows that immunotherapy targeting cytotoxic T-lymphocyte antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1) and programmed death ligand-1 (PD-L1) can significantly improve patient survival of metastatic melanoma, urothelial carcinoma, prostate cancer, non-small cell lung cancer, and other malignant tumors 98-101.