Abstract:The tyrosine hydroxylase (TH) gene is expressed exclusively in cells and neurons that synthesize and releasel‐DOPA or catecholamines. To further understand the molecular genetic mechanisms that regulate this cell‐type specific expression, a chimeric gene was prepared by linking 3.6 kb of the 5′ flanking DNA of the mouse TH gene, including the +1 initiation site for transcription, to anE. coliβ‐galactosidase reporter. This fusion gene (TH3.6LAC) was used to prepare transgenic mice, and the tissue distribution of expression of TH3.6LAC was determined by the measurement of β‐galactosidase enzymatic activity and/or by the detection of the transcription product of the chimeric gene by RNase protection assays. In two separate founder lines, TH3.6LAC expression was observed in every region of the brain that was examined, including the olfactory bulb, brainstem, cerebellum, diencephalon, hippocampus, striatum, and cerebral cortex. Expression of TH3.6LAC was observed in the adrenal gland of one founder line but not in the other. TH3.6LAC activation was undetectable in peripheral organs that were examined, including the liver, heart, salivary gland, kidney, lung, and spleen. Although 3.6 kb of the 5′ regulatory DNA of the mouse TH gene is sufficient to activate the TH fusion gene in the mouse, it is not enough to restrict its expression to catecholam