The goal of cancer immunotherapy is to harness the immune system to recognize and destroy tumor cells, potentially producing durable responses that may translate into curative outcomes in patients with metastatic cancers. Results from randomized clinical trials have established immune checkpoint inhibitors as the most successful class of immunotherapies to date. These include monoclonal antibodies that reinvigorate T-cell responses by blocking cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed cell death-1 (PD-1), 2 coinhibitory receptors that regulate T-cell activation.