We have previously demonstrated that the pineal hormone melatonin exerts an inhibitory effect on the invasive properties of human breast cancer cells (Mao et al., 2010).Follow-up studies demonstrate that melatonin administration (0.01-10 nM) enhances E-eadherin protein expression and represses vimentin expression in MCF-7 and MCF-7/caSrc human breast cancer cells.Collectively, these results suggest that melatonin may suppress breast cancer cell invasion by blocking epithelial-to-mesenchymal transition (EMT), which increases cell motility and invasiveness.