CXCR4 and breast cancer: Previous findings have identified estrogen as a positive regulator of the CXCL12/SDF‐1 gene in breast cancer cells (Hall and Korach, 2003; Sauve et al., 2009), and we have described that such regulation is part of a positive autocrine feedback loop involving CXCR4 and estrogen receptors to promote cell growth (Sauve et al., 2009).