MicroRNAs (miRNAs) are involved in an abundant class of post-transcriptional regulation activated through binding to the 3() -untranslated region (UTR) of mRNAs. The current wealth of mammalian miRNA genes results mostly from genomic duplication events. Many of these events are located within introns of transcriptional units. In order to better understand the genomic expansion of miRNA genes, we investigated the distribution of intronic miRNAs.