MirGeneDB is a database of microRNA genes that have been validated and annotated as described in 'A Uniform System for the Annotation of Vertebrate microRNA Genes and the Evolution of the Human microRNAome'.* The initial version contained 1,434 microRNA genes for human, mouse, chicken and zebrafish. Version 2.0 contains more than 10,000 genes from 45 organisms representing nearly every major metazoan group, and these microRNAs can be browsed, searched and downloaded