Here we describe the expression pattern of a previously unknown mouse gene mPet-1. The isolated cDNA codes for an ETS-domain transcription factor of 237 amino acids in length, which is localized to the nucleus. mPet-1 is a member of the winged helix transcription factor gene family like its rat homologue Pet-1 and the human homologue FEV. The start ATG of mPet-1 and the size of the predicted protein are identical to the human FEV. The mPet-1 protein is clearly smaller since it lacks the first 103 N-terminal amino acids of rat Pet-1. mPet-1 is expressed in central serotonergic (5-hydroxytryptaminergic) neurons located in the mes-/metencephalic raphe nuclei from E11 on until adulthood. In these regions mPet-1 expression co-localizes precisely with the serotonin transporter (Sert), which it initially precedes. Interestingly, mPet-1 was not found in neurons transiently expressing Sert.