NEUROGENIN1 IS ESSENTIAL FOR THE DETERMINATION OF NEURONAL PRECURSORSFOR PROXIMAL CRANIAL SENSORY GANGLIA

The NEUROGENINS (NGNs) are neural-specific basic helix-loop-helix (bHL H) transcription factors. Mouse embryos lacking ngn1 fail to generate the proximal subset of cranial sensory neurons. ngn1 is required for t he activation of a cascade of downstream bHLH factors, including Neuro D, MATH3, and NSCL1. ngn1 is expressed by placodal ectodermal cells an d acts prior to neuroblast delamination. Moreover, NGN1 positively reg ulates the Delta homolog DLL1 and can be negatively regulated by Notch signaling. Thus, ngn1 functions similarly to the proneural genes in D rosophila. However, the initial pattern of ngn1 expression appears to be Notch independent. Taken together with the fact that ectopic ngn1 e xpression can convert ectodermal cells to neurons in Xenopus (Ma et al ., 1996), these data and those of Fode et al. (1998 [this issue of Neu ron]) identify ngns as vertebrate neuronal determination genes, analog ous to myoD and myf5 in myogenesis.