It is intended to develop an RNAi molecule that permits the expression of wild-type or desired alleles and can selectively and effectively suppress only the expression of a particular dominant allele and to provide a guideline for design thereof and an agent for suppressing the expression of a dominant allele, comprising the RNAi molecule as an active ingredient. The present invention provides an agent for suppressing the expression of a dominant allele, comprising an RNAi molecule having a structure obtained by assigning a dominant point mutation in the targeted dominant allele as a standard point, setting a base length from the standard point to the 5' end to a predetermined length, and introducing one mismatch base different from the corresponding base in the sequence of the target gene, to a predetermined position downstream from the standard point.
