The ability to generate mice containing null mutations in any cloned gene promises new insights into the molecular control of mammalian neural crest development. This approach has recently been applied to MASH-1, a transcription factor in the bHLH family that is a mammalian homologue of the Drosophila proneural genes achaete-scute. In wild-type embryos, this gene is expressed early in the development of the autonomic nervous system, in apparent precursors of sympathetic, parasympathetic, and enteric neurons (as well as in restricted regions of the central nervous system). A null mutation in the MASH-1 gene eliminates sympathetic and parasympathetic neurons and enteric neurons of the foregut (esophagus); however, enteric neurons of the stomach and hindgut are only partially affected. Analysis with other markers indicates that the mutation acts after neural crest cells have localized in the anlagen of the autonomic nervous system to prevent neuronal differentiation. The differentiation of autonomic glia appears unaffected. Thus, MASH-1 provides one of the most specific mutations affecting neural development in mammals, as well as a valuable marker to study the early segregation of neural crest cell lineages.