The spatiotemporal control of gene expression is crucially important during development, and microRNAs (miRNAs; short RNA molecules that silence complementary mRNA sequences) are thought to fine-tune the expression of developmentally important genes. Here, Ramesh Shivdasani and colleagues report that specific miRNAs influence mouse stomach organogenesis by regulating the expression of the mesenchymal transcription factor Barx1 (see p. 1081). Barx1 controls stomach morphogenesis and helps to specify the stomach-specific epithelium. However, Barx1 levels in the stomach decline sharply after epithelial specification. The researchers show that depletion of the miRNA-processing enzyme Dicer in cultured stomach mesenchymal cells increases Barx1 levels and that conditional Dicer gene deletion in mice disrupts stomach development. They identify miR-7a and miR-203 as regulators of Barx1 expression and show that these miRNAs repress Barx1 expression in the developing stomach by binding to the Barx1 3′ untranslated region. Barx1 downregulation by miRNAs in the mouse embryonic stomach might thus be an example of a widely used mechanism for modulating gene expression during development.