We investigated a gene expression imaging method to examine the level of therapeutic gene expression in the cerebellum. Using a human immunodeficiency virus derived lentivial vector, we expressed the dopamine D(2) receptor (D(2)R) as a reporter protein to mouse cerebellar Purkinje cells. Biodistribution and ex vivo autoradiography studies were performed by giving [(125)I]5-iodo-7-N-[(1-ethyl-2-pyrrolidinyl)methyl]carboxamide-2,3-dihydrobenzofuran ([(125)I]IBF) (1.85 MBq), as a radioactive D(2)R ligand, to model mice expressing the D(2)R with an HA tag (HA-D(2)R) in the cerebellum. In this study, [(125)I]IBF was bound to the D(2)R expressed in the cerebellum of the model mice selectively. Immunostaining was performed to confirm the HA-D(2)R expression in the cerebellum of the model mice. A significant correlation (r=0.900, P<0.001) between areas that expressed HA-D(2)R by immunostaining and areas in which [(125)I]IBF accumulated by the ex vivo autoradiograms was found. These results indicated that radioiodinated IBF is useful as a reporter probe to detect D(2)R reporter gene expression, which can be used for monitoring therapeutic gene expression in the cerebellum.