Abstract: The present invention relates to a method for inhibiting expression of a target gene, which comprises introducing a cell, tissue, or individual organism with a double-stranded polynucleotide comprising DNA and RNA having a substantially identical nucleotide sequence with at least a partial nucleotide sequence of the target gene.

Abstract: The present invention provides a polynucleotide that not only has a high RNA interference effect on its target gene, but also has a very small risk of causing RNA interference against a gene unrelated to the target gene. A sequence segment conforming to the following rules (a) to (d) is searched from the base sequences of a target gene for RNA interference and, based on the search results, a polynucleotide capable of causing RNAi is designed, synthesized, etc.: (a) The 3? end base is adenine, thymine, or uracil, (b) The 5? end base is guanine or cytosine, (c) A 7-base sequence from the 3? end is rich in one or more types of bases selected from the group consisting of adenine, thymine, and uracil, and (d) The number of bases is within a range that allows RNA interference to occur without causing cytotoxicity.