PDCD1 and metastatic melanoma: More recently, immunotherapy (i.e., antibodies blocking programmed cell death protein 1 (PD-1) and/or cytotoxic T-lymphocyte-associated protein-4 (CTLA-4)) has changed the landscape of systemic therapy for metastatic solid tumors (e.g., metastatic melanoma, non-small-cell lung cancer, and genitourinary cancers), thereby improving the survival of cancer patients who were resistant to traditional therapies [3,4,5,6].