It is intended to develop an RNAi molecule that can selectively and effectively suppress only the expression of a particular dominant mutant gene, while permitting the expression of the wild-type gene or a desired mutant gene, and to provide a design method thereof and an agent for suppressing the expression of a dominant mutant gene, comprising the RNAi molecule as an active ingredient. The present invention provides an agent for suppressing the expression of a dominant mutant gene, comprising an RNAi molecule against the dominant mutant gene generating a point of discontinuity on a transcript, the RNAi molecule having a predetermined length set as a base length from a base 3 flanking the point of discontinuity on a sense strand region to the 3 end of the sense strand region.
