Here, we report that the bivalent second mitochondria-derived activator of caspases (Smac) mimetic, SM-16423,24, which was developed to kill cancer cells by degrading both cIAP (cellular inhibitor of apoptosis) and XIAP (X-linked inhibitor of apoptosis) proteins23,24, eliminates early stage metastatic BC and significantly reduces the progression of advanced bone and lung metastases from the triple-negative human breast cancer cell line, MDA-MB-231, in a mouse model. The gene discussed is XIAP; the disease is breast cancer.