Breast cancer stem cells are regulated by mesenchymal stem cells via cytokine networks. Breast cancer cells secrete IL6, which recruits bone marrow–derived mesenchymal stem cells and induces CXCL7 secretion by these cells. CXCL7 in turn induces the production of multiple cytokines including IL8 and IL6, which completes a positive feedback loop and stimulates the self-renewal of breast cancer stem cells driving tumor growth and metastasis. For details, see the article by Liu and colleagues on page 614 of this issue.