The process of active nuclear protein transport is mediated by the nuclear localization signal (NLS). An NLS-containing karyophile forms a stable complex, termed the nuclear pore-targeting complex, to target nuclear pores. The alpha-subunit of the complex (importin-alpha) binds to the NLS and the beta-subunit (importin-beta) carries the alpha-subunit, bound to the NLS substrate, into the nucleus. To date, five mouse alpha-subunits have been identified and classified into three subfamilies (alpha-P, alpha-Q, and alpha-S). The expression of these alpha-subunits and the beta-subunit in various adult mouse tissues was examined by immunoblotting and immunohistochemistry using antibodies specific for each subfamily of the alpha-subunit or the beta-subunit. The beta-subunit was found to be ubiquitously expressed, whereas each subfamily of the alpha-subunit showed a unique expression pattern in various tissues, especially in brain and testis. In brain, the expression of alpha-P was not observed, whereas alpha-S was significantly expressed in Purkinje cells, and pyramidal cells of the hippocampus and cerebral cortex. In testis, alpha-P was expressed predominantly in primary spermatocytes, whereas alpha-Q was found mainly in Leydig cells. Expression of alpha-S was detected in almost all cells in convoluted seminiferous tubules and Leydig cells to a similar extent. These results suggest that nuclear protein import may be controlled in a tissue-specific manner by alpha-subunit family proteins.