Piwi-interacting RNAs (piRNAs), which are specifically expressed in the germ cells of Drosophila melanogaster, are small non-coding RNAs that have complementary sequences to transposons. The transfer of transposons results in genome mutations. If they occur in germ cells, it is disadvantageous to the survival of the species and a system that can prevent such mutations is necessary. piRNAs perform a central role in that system. In our laboratory, to understand the piRNA-derived transposon regulation system, we established and studied a line of ovarian somatic cells (OSCs). In OSCs, piRNAs form a piRISC (piRNA-induced silencing complex), which is then transported to the nucleus to regulate transposons in the transcription phase. Our previous results indicated that it is essential to piRISC nuclear transport that the protein Piwi binds to piRNAs. However, the mechanisms by which that system operates are still unclear. In the present study, we examined the role of the nuclear transport factor importin α. D. melanogaster has three subtypes of importin α (Impα1, Impα2, Impα3). When any one of these Impα proteins is knocked down, the nuclear transport of Piwi is prevented. Each Impα binds to the nuclear localization signal region of Piwi. These results strongly indicate that the nuclear transport of piRISC involves the Impα proteins. We also have preliminary data showing that Piwi that does not bind to piRNA normally, but rather binds to Impα in vitro. Therefore, our data suggest that only Piwi which has bound to piRNAs is capable of binding Impα in OSCs; we are continuing to research this interaction.