The activating transcription factor (ATF) family comprises a group of basic region-leucine zipper (bZIP) proteins, which have roles in the development of species as diverse as insects and mammals. Here we describe two novel mRNAs encoding a single, 30-kDa mouse polypeptide, designated mouse ATF5, which is 58% identical to mouse ATF4 in the carboxy-terminal bZIP region. Both transcripts harbor highly complex 5' untranslated regions that impede translation of the ATF5 open reading frame. The mouse and human ATF5 loci consist of at least four exons contained within 5 kb of genomic sequence. During mouse embryonic development, expression of Atf5 is pronounced at the late gestational period and appears to be confined to cells of the neuronal layers of the olfactory epithelium and vomeronasal organ. This suggests a role for ATF5 in odorant sensory neuron differentiation.