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 α-subunit of the complex (importin-α) binds to the NLS and the β-subunit (importin-β-) carries the α-subunit, bound to the NLS substrate, into the nucleus. To date, five mouse α-subunits have been identified and classified into three subfamilies (α-P, α-Q, and α-S). The expression of these α-subunits and the β-subunit in various adult mouse tissues was examined by immunoblotting and immunohistochemistry using antibodies specific for each subfamily of the α-subunit or the β-subunit. The β-subunit was found to be ubiquitously expressed, whereas each subfamily of the α-subunit showed a unique expression pattern in various tissues, especially in brain and testis. In brain, the expression of α-P was not observed, whereas α-S was significantly expressed in Purkinje cells, and pyramidal cells of the hippocampus and cerebral cortex. In testis, α-P was expressed predominantly in primary spermatocytes, whereas α-Q was found mainly in Leydig cells. Expression of α-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 α-subunit family proteins.