In the mouse olfactory system, individual olfactory sensory neurons expresses only one single olfactory receptor (OR) gene from more than a thousand OR genes dispersed throughout the genome. The regulatory DNA sequences that enable OR gene choice are unknown. We searched the promoter regions of a large number of OR genes for common cis-regulatory sequences that may be involved in their gene regulation. First, to obtain the promoter regions of different OR genes we generated cDNAs with complete 5'-untranslated regions using RLM-RACE and degenerate primers that recognize conserved sequences in olfactory receptor genes. Then these cDNAs were aligned with the mouse genome and the precise positions of the transcriptional start sites (TSSs) were determined for a total of 198 different OR genes. The promoter sequences were  retrieved and browsed for common elements. We found four different  motifs that resemble O/E binding sites concentrated within 200 pb upstream of the corresponding TSSs. Interestingly, 170 out of the 198 OR genes analyzed have at least one of the four motifs in their promoter sequences. These four O/E-like sites specifically interact with nuclear proteins extracted from olfactory epithelium, but not with nuclear proteins extracted from brain or liver in gel shift assays. For the first time, we demonstrated that several OR genes have potential O/E- like sites in their promoter regions that may play a role in the olfactory receptor gene choice.