Mouse odorant receptors (ORs) are encoded by >1000 genes dispersed throughout the genome. Each olfactory neuron expresses one single OR gene, while the rest of the genes remain silent. The mechanisms underlying OR gene expression are poorly understood. Here, we investigated if OR genes share common cis-regulatory sequences in their promoter regions. We carried out a comprehensive analysis in which the upstream regions of a large number of OR genes were compared. First, using RLM-RACE, we generated cDNAs containing the complete 5'-untranslated regions (5'-UTRs) for a total number of 198 mouse OR genes. Then, we aligned these cDNA sequences to the mouse genome so that the 5' structure and transcription start sites (TSSs) of the OR genes could be precisely determined. Sequences upstream of the TSSs were retrieved and browsed for common elements. We found DNA sequence motifs that are overrepresented in the promoter regions of the OR genes. Most motifs resemble O/E-like sites and are preferentially localized within 200 bp upstream of the TSSs. Finally, we show that these motifs specifically interact with proteins extracted from nuclei prepared from the olfactory epithelium, but not from brain or liver. Our results show that the OR genes share common promoter elements. The present strategy should provide information on the role played by cis-regulatory sequences in OR gene regulation.