MicroRNAs (miRNAs) are a class of short ( approximately 22-nt) noncoding RNA molecules that downregulate expression of their mRNA targets. Since their discovery as regulators of developmental timing in Caenorhabditis elegans, hundreds of miRNAs have been identified in both animals and plants. Here, we report a technique for visualizing detailed miRNA expression patterns in mouse embryos. We elucidate the tissue-specific expression of several miRNAs during embryogenesis, including two encoded by genes embedded in homeobox (Hox) clusters, miR-10a and miR-196a. These two miRNAs are expressed in patterns that are markedly reminiscent of those of Hox genes. Furthermore, miR-196a negatively regulates Hoxb8, indicating that its restricted expression pattern probably reflects a role in the patterning function of the Hox complex.