cash4, a novel achaete-scute homolog induced by Hensen's node during generation of the posterior nervous system.

In vertebrate embryos, the precursor cells of the central nervous system (CNS) are induced by signaling from the organizer region. Here we report the isolation of a novel vertebrate achaete-scute homolog, cash4, which is expressed in the presumptive posterior nervous system in response to such signaling. cash4 is first expressed in epiblast cells flanking the late-phase organizer (Hensen's node), which retains its ability to induce cash4 during regression to the caudal end of the embryo. We show that these node-derived signals can be mimicked in vivo by the activity of fibroblast growth factor (FGF). We demonstrate that cash4 can substitute for the achaete/scute genes in the fly and that it also has proneural activity in vertebrate embryos. Together these results suggest that cash4 functions as a proneural gene downstream of node-derived signals (including FGF) to promote the formation of the neural precursors that will give rise to the posterior CNS in the chick embryo.