To define the role of melatonin in breast cancer cell invasion and metastasis, we used three invasive breast cancer cell clones that were derived from the parental MCF-7 cells - the MCF-7/6, MCF-7/Her2.1, and MCF-7/CXCR4 cells - instead of using the poorly invasive MCF-7 breast cancer cells. The gene discussed is CXCR4; the disease is breast cancer.