MSH2 and Huntington disease: 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.