FXN and fragile X syndrome: Indeed, crossing of mice harbouring expandable triplet repeats with mice lacking the MMR protein MutS homologue 2 (MSH2) or MutS homologue 3 (MSH3) attenuates expansion in the 5′-CAG-3′ repeats in the human HD gene9, 10, 11, 12, 13, 14, 15, 16, the 5′-CTG-3′ repeats in the 3′-untranslated region of the human myotonic dystrophy 1 protein kinase transgene17, 18, 19, 20, the 5′-GAA-3′ repeats in the FXN gene in Friedreich's ataxia (FRDA)21 and the 5′-CGG-3′ repeats in the fragile mental retardation gene in fragile X syndrome (FXS)22.