We found that breast cancer cells and bone cells influence each other in the bone microenvironment by producing CCL20; the RANKL/OPG ratio was increased in both MDA-MB-231 cells and human osteoblast hFOB1.19 cells exposed to CCL20, and osteoblast-derived CCL20 promoted breast cancer cell invasiveness. The gene discussed is CCL20; the disease is breast cancer.