As small non-coding regulatory RNAs, microRNAs are capable of silencing gene expression by translational repression or mRNA degradation. Accumulating evidence indicates that deregulation of microRNAs is often associated with human malignancies and suggests a causal role of microRNAs in neoplasia, presumably because microRNAs can function as oncogenes or tumor suppressors. Among them, miR-205 is significantly underexpressed in breast tumors compared with matched normal breast tissue although miR-205 has been shown to be upregulated in some other type of tumors. Furthermore, breast cancer cell lines, including MCF-7 and MDA-MB-231, express a lower level of miR-205 than the non-malignant MCF-10A cells. Ectopic expression of miR-205 significantly inhibits cell proliferation and anchorage-independent growth as well as cell invasion. These findings establish the tumor suppressive role of miR-205, which is probably through direct targeting of oncogenes such as ErbB3 and Zeb1. Therefore, miR-205 may serve as a unique therapeutic target for breast cancer.