MicroRNA (miRNA)-mediated gene silencing has recently emerged as a major mechanism of gene expression regulation during development in a variety of species. Little is known, however, about the presence of components of miRNA machinery in mammalian organogenesis. In this study, we report that members of the Argonaute (Ago) gene family are expressed in restricted of the day 11.5 and 14.5 embryo, including the brain, neural tube, limb, lungs, and hair follicles. In the developing lung, we found expression of Ago1 and Ago2 localized to branching regions, in distal epithelium and mesenchyme, respectively. These were sites undergoing the most dynamic changes in gene expression and rapid remodeling. We show that Ago1 transcripts are enriched in neural structures at these stages, consistent with the reported role of Drosophila Ago1 in the development of the central nervous system. Our results suggest a role for miRNAs in organogenesis. (c) 2005 Elsevier Inc. All rights reserved.