The mixture of CXCL4 and CXCL12 or the obligate CXCL4•CXCL12 heterodimer inhibited CXCL12-induced migration of MDA-MB-231 breast cancer cells, increased cytoplasmic Ca2+ release and activated downstream signaling of CXCR4 but not CXCR3, highlighting its role in limiting cancer progression. Here, CXCL12 is linked to breast cancer.