The mammalian Olf1/EBF (O/E) family of repeated helix-loop-helix (rHLH)transcription factors has been implicated in olfactory system gene regulation,nervous system development and B-cell differentiation. Ebf(O/E1) mutant animals showed defects in B-cell lineage and brain regions where it is the only O/E family member expressed, but the olfactory epithelium appeared unaffected and olfactory marker expression was grossly normal in these animals. In order to further study the mammalian O/E proteins,we disrupted O/E2 and O/E3 genes in mouse and placed tau-lacZ and tau-GFP reporter genes under the control of the respective endogenous O/E promoters. Mice mutant for each of these genes display reduced viability and other gene-specific phenotypes. Interestingly, both O/E2 and O/E3 knockout mice as well as O/E2/O/E3 double heterozygous animals share a common phenotype:olfactory neurons (ORN) fail to project to dorsal olfactory bulb. We suggest that a decreased dose of O/E protein may alter expression of O/E target genes and underlie the ORN projection defect.