The microRNA miR-196 acts upstream of Hoxb8 and Shh in limb development

MicroRNAs ( miRNAs) are an abundant class of gene regulatory molecules ( reviewed in refs 1, 2). Although computational work indicates that miRNAs repress more than a third of human genes(3), their roles in vertebrate development are only now beginning to be determined. Here we show that miR- 196 acts upstream of Hoxb8 and Sonic hedgehog ( Shh) in vivo in the context of limb development, thereby identifying a previously observed but uncharacterized inhibitory activity that operates specifically in the hindlimb. Our data indicate that miR- 196 functions in a fail- safe mechanism to assure the fidelity of expression domains that are primarily regulated at the transcriptional level, supporting the idea that many vertebrate miRNAs may function as a secondary level of gene regulation.