The lineage of olfactory neurons has been relatively well characterized at the cellular level, but the genes that regulate the proliferation and differentiation of their progenitors are currently unknown. In this study, we report the isolation of a novel murine gene, Math4C/neurogenin1, which is distantly related to the Drosophila proneural gene atonal. We show that Math4C/neurogenin1 and the basic helix-loop-helix gene Mash1 are expressed in the olfactory epithelium by different dividing progenitor populations, while another basic helix-loop-helix gene, NeuroD, is expressed at the onset of neuronal differentiation. These expression patterns suggest that each gene marks a distinct stage of olfactory neuron progenitor development, in the following sequence: Mash1>Math4C/neurogenin1>NeuroD. We have previously reported that inactivation of Mash1 function leads to a severe reduction in the number of olfactory neurons. We show here that most cells in the olfactory epithelium of Mash1 mutant embryos fail to express Math4C/neurogenin1 or NeuroD. Strikingly, a subset of progenitor cells in a ventrocaudal domain of Mash1 mutant olfactory epithelium still express Math4C/neurogenin1 and NeuroD and differentiate into neurons. Cells in this domain also express Math4A/neurogenin2, another member of the Math4/neurogenin gene family, and not Mash1. Our results demonstrate that Mash1 is required at an early stage in the olfactory neuron lineage to initiate a differentiation program involving Math4C/neurogenin1 and NeuroD. Another gene activates a similar program in a separate population of olfactory neuron progenitors.