PROBLEM TO BE SOLVED: To provide RNAi-inducing nucleic acid molecules having cell penetrating ability.SOLUTION: The invention provides an RNAi-inducing double-stranded nucleic acid molecule having cell-penetrating ability, where the nucleic acid molecule comprises a first strand comprising a region complementary to a target nucleic acid and a second strand that forms a complementary bond with the first strand, and the phosphate backbone of at least one nucleotide in the nucleic acid molecule is substituted with phosphorothioate or phosphorodithioate, and the nucleic acid molecule has a lipophilic compound conjugated thereto.EFFECT: The invention provides a nucleic acid molecule of a novel structure having a high gene silencing efficacy while having cell penetrability not requiring a separate intracellular delivery vehicle, as well as a method for inhibiting expression of a target gene using it. The nucleic acid molecule of the invention can be utilized in siRNA treatment of cancer or viral infection in place of conventional siRNA molecules.SELECTED DRAWING: Figure 12【課題】細胞内貫通能を持つＲＮＡｉ誘導用核酸分子の提供。【解決手段】標的核酸と相補的な領域を含む第１鎖と、前記第１鎖と相補的結合を形成する第２鎖で構成されるＲＮＡｉ誘導用二本鎖核酸分子において、前記核酸分子に含まれている少なくとも一つのヌクレオチドのホスフェートバックボーンがホスホロチオエート又は、ホスホロジチオエートで置換されていて、親油性化合物が結合されている、細胞貫通能を有するＲＮＡｉ誘導用核酸分子。【効果】優秀な目的遺伝子抑制効率を持ちながらも別途の細胞伝達体なしに細胞内貫通能を持つ新しい構造の核酸分子及びこれを利用した目的遺伝子の発現抑制方法で、従来のｓｉＲＮＡ分子に代えてｓｉＲＮＡを利用した癌やウィルス感染治療などに活用することができて有用である。【選択図】図１２
