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MicroRNAs (miRNAs) contains 18-24 nucleotides,which are often linked as clusters transcribed from genomic DNAs.The miR-17~92 is the most extensively studied cluster.This cluster and its paralogs play important roles in cancers because of their ability to repress expression of many tumor-associated proteins.Transcription of this cluster is trans-activated by the oncogene c-Myc.Comprehensive microarray analysis has revealed deletion of miR-17~92 in some ovarian cancers,breast cancers and melanomas.Overexpression of miR-17~92 promotes cell proliferation and reduces apoptosis by regulating cell cycle progression.Inhibition of miR-17 and miR-20a induces apoptosis,while inhibition of miR-18a and miR-19a in the same cluster has no apparent effect.These studies indicate functional distinctions among miRNAs in the miR-17~92 cluster.Recent studies show that the miR-17~92 cluster plays an important role in tissue development.However,the exact role of each miRNA in the cluster is not clear.