Estimated 600 metastatic triple-negative breast cancer (mTNBC) patients with central determination of PD-L1 tumor status who has previously received either one or two prior systemic treatments for metastatic setting will receive pembrolizumab 200 mg intravenously every 3 weeks or receive capecitabine, eribulin, gemcitabine, and vinorelbine per physician’s choice. The gene discussed is CD274; the disease is neoplasm.