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MicroRNAs (miRNAs) are small, noncoding RNAs with regulatory functions, which play an important role in many human diseases, including cancer. An emerging number of studies show that miRNAs can act either as oncogenes or as tumor suppressor genes or sometimes as both. Germline, somatic mutations and polymorphisms can contribute to cancer predisposition. miRNA expression levels have diagnostic and prognostic implications, and their roles as anticancer therapeutic agents is promising and currently under investigation.