Breast tumor cells and ES cells secrete cytokines and chemokines to their microenvironment, however, breast cancer cells secrete a significantly higher level of soluble factors, which is correlated with tumor metastasis (e.g., M-CSF, OSM, MIP-2/CXCL-2, MMP-9, TIMP-1). This evidence concerns the gene CXCL2 and breast cancer.