Examples of Drosophila miRNA gene clusters. In this figure, pre-miRNAs are represented by rectangles and the arm that gives rise to the mature miRNA is colored. (a) The largest miRNA cluster was previously identified by Tuschl and colleagues ; we identified and experimentally verified a new member of this cluster, mir-286. A second conserved hairpin was found (light gray box), but its expression was not seen. Of the seven genes in this cluster, only mir-286 is conserved in Anopheles (ano). Note also that this cluster contains both related miRNA genes (mir-6-1, -2, -3 and the K-box antisense gene mir-5, yellow), as well as unrelated miRNA genes (black). (b) A second example of rapid miRNA gene evolution. The Anopheles genome contains four members of the mir-2/mir-13 family, which are all located in a single cluster. In contrast, drosophilid genomes contain eight members of this family, located at four distinct genomic locations on three different chromosomes. (c) A cluster of putative developmental regulators. let-7 and mir-125 are orthologous to the genetically characterized genes let-7 and lin-4 in C. elegans. A similar gene cluster exists in Anopheles, although mir-100 is separated from the other two by several kilobases (not shown). (d) Other examples of miRNA clusters. Note that, as is the case for the other clusters shown, miRNA clusters can contain related genes (yellow), but appear to be as likely to contain unrelated genes (black).