Surprisingly, however, the eukaryotic mismatch recognition complex, MSH2–MSH3, not only fails to act as a guardian of the genome at the long disease-length trinucleotide repeat (TNR) tracts but also causes expansion, the lethal mutation underlying Huntington's disease (HD) as well as at least 30 other fatal diseases4, 5, 6, 7, 8. This evidence concerns the gene MSH3 and Huntington disease.