An agent for selectively suppressing the expression of a dominant allele while allowing expression of wild-type or desired alleles and methods for using the agent are described. The RNAi agent has a structure obtained by assigning a dominant point mutation in the targeted 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 differing from the target sequence to a predetermined position downstream from the standard point.