Choroid plexuses - secretory organs in the brain that produce cerebrospinal fluid (CSF) and that function as the blood-CSF barrier - regulate the internal environment of the brain. Little is known, however, about the development and growth of these crucial organs. On p. 2535, Huang and co-workers begin to remedy this situation by reporting that sonic hedgehog (Shh) signalling regulates a novel epithelial progenitor domain in the mouse hindbrain choroid plexus (hChP). Shh is strongly expressed in the epithelium of the differentiated hChP but the researchers now identify a distinct epithelial domain in the developing hChP that does not express Shh. Instead, this domain, which adjoins the lower rhombic lip (a germinal zone or cellular birth place), displays Shh signalling. The researchers show that this Shh target field contributes directly to the development of the hChP epithelium by acting as a progenitor domain. Finally, by analysing conditional Shh mouse mutant embryos, they provide evidence that tissue-autonomous Shh production and signalling regulates the growth of the hChP throughout development.