FMR1 and fragile X-associated tremor/ataxia syndrome: Whereas it is possible that the cellular heterogeneity of the human CNS may mask toxic levels of FMR1 mRNA, other hypotheses, including an inappropriate DNA damage response, mitochondrial stress, and polyglycine-containing peptide accumulation, have been put forth as additional hypotheses to explain the pathophysiology of FXTAS (11, 27, 28).