MicroRNAs (miRs) are small endogenous RNAs that have important biological functions (see for review, Bartel1 and Ambros2). The miRs are 17–25bp, generated from longer noncoding RNAs in step-wise processes. First, primary miRs are transcribed from genomic DNA and then truncated to produce B70 bp of RNAs, called precursor miRs in the nucleus. Second, the precursor miRs are further cut to become mature miRs in the cytoplasm. Although the detailed mechanism remains unclear, the miRs can function as translation repressors through binding to 30 untranslation regions of target messenger RNAs. Since many miRs highly conserve nucleotide alignments between distantly related species, it has been suggested that the small RNAs have fundamental biological roles. Accumulated results have proven that miRs participate in diverse biological processes. Examples include developmental timing, cellular