To develop a cell type-specific and temporal regulation system of gene targeting in the cerebellum, we used the NMDA-type glutamate receptor GluR⑀3 subunit gene and Cre recombinaseprogesterone receptor fusion (CrePR) gene in combination. Injection of the CrePR gene placed under the control of the 10 kb 5Ј region of the GluR⑀3 gene into C57BL/6 eggs yielded the ECP25 line that strongly expressed the CrePR mRNA selectively in the granule cells of the cerebellum. Using a transgenic mouse carrying

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... reporter gene for Cre-mediated recombina-tion, we showed that antiprogestins could induce the recombinase activity of CrePR protein in the cerebellar granule cells of the ECP25 line. Thus, the established mouse line will provide a valuable tool to investigate the mechanism of cerebellar function by manipulating molecules in the temporally regulated and granule cell-specific manner.