Nucleotide variations, including SNPs, in the coding regions of disease genes are important targets for RNAi treatment, which is a promising medical treatment for intractable diseases such as triplet repeat diseases. However, the identification of such nucleotide variations and the design of siRNAs conferring disease allele-specific RNAi are quite difficult. In this study we developed a pull-down method to rapidly identify coding SNP (cSNP) haplotypes of triple repeat, disease-causing alleles, and we demonstrated disease allele-specific RNAi that targeted cSNP sites in mutant Huntingtin alleles, each of which possessed a different cSNP haplotype. Therefore, the methods presented here allow for allele-specific RNAi knockdown against disease-causing alleles by using siRNAs specific to disease-linked cSNP haplotypes, and advanced progress toward tailor-made RNAi treatments for triplet repeat diseases.