We have characterized genomic structure and transcription of a murine odorant receptor (OR) gene, MOR23. This gene has two distinct transcriptional initiation regions, which are differentially utilized in the olfactory and testicular cells. We have found that the MOR23 gene has an intron within the 5'-untranslated region upstream of the coding sequence. In olfactory cells, transcription of the gene is initiated in the region upstream of this intron, while in testicular cells, transcription starts within the intron. Differential initiation of transcription is discussed in the context of tissue-specific expression of the OR genes.