Over the past year, our research efforts have focused on the generality of the results we found in human induced pluripotent stem cells derived from patients with the neurodegenerative disease Friedreich’s ataxia (FRDA). FRDA is one of the trinucleotide repeat (TNR) diseases, and our major previous finding was that the GAA•TCC trinucleotide repeats that cause FRDA expand during isolation and propagation of FRDA hiPSCs. This expansion was shown to be dependent on enzymes that are involved in the repair of mismatches in the human genome. To extend these studies, we have now focused on hiPSCs from the related TNR diseases myotonic dystrophy, Huntington’s disease and Fragile X syndrome. Myotonic dystrophy type 1 (DM1) is an inherited dominant muscular dystrophy caused by expanded CTG•CAG triplet repeats in the 3’ UTR of the DMPK1 gene, which produces a toxic gain-of-function CUG RNA. It has been shown that the severity of disease symptoms, age of onset and progression are related to the length of the triplet repeats. However, the mechanism(s) of CTG•CAG triplet-repeat instability is not fully understood. Human induced pluripotent stem cells (iPSCs) were generated from DM1 and Huntington’s disease (HD) patient fibroblasts. We isolated 41 iPSC clones from DM1 fibroblasts, all showing different CTG•CAG repeat lengths, thus demonstrating somatic instability within the initial fibroblast population. During propagation of the iPSCs, the repeats expanded in a manner analogous to the intergenerational expansion observed in DM1 patient families. The correlation between repeat length and expansion rate identified the interval between 57 and 126 repeats as being an important length threshold where expansion rates dramatically increased. Moreover, longer repeats showed faster triplet-repeat expansion. The relatively short repeats in the gene responsible for Huntington’s disease are below this threshold and hence do not expand in the iPSCs. The overall tendency of triplet repeats to expand ceased on differentiation into differentiated embryoid body or neurospheres. The mismatch repair components MSH2, MSH3 and MSH6 were highly expressed in iPSCs compared to fibroblasts, and only occupied the DMPK1 gene harboring longer CTG•CAG triplet repeats. In addition, shRNA silencing of MSH2 impeded CTG•CAG triplet-repeat expansion. We have also generated hiPSC lines from seven male subjects clinically diagnosed with fragile X syndrome. These hiPSCs have been thoroughly characterized with respect to pluripotency, DNA methylation status at the FMR1 gene, CGG repeat length, FMR1 expression and neuronal differentiation. The information gained from these studies provides new insight into a general mechanism of triplet repeat expansion in iPSCs.