Micro-RNAs (miRs) are a recently described class of genes, encoding small non-coding RNA molecules, which primarily act by down-regulating the translation of target mRNAs. miRs are involved in a range of normal physiological processes, notably differentiation and cell type determination. It has become apparent that they are also key factors in cancer, playing both oncogenic and tumour-suppressing roles. We discuss here what is known of miR biology in the normal breast, and of their emerging roles in breast cancer.
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