While in non-carriers the decrease in citrate synthase activity was mirrored by a decline in Complex IV activity (Spearman r = 0.6002, p = 0.0031) pointing to an overall decline in mitochondrial mass, no statistically significant correlation was observed between these outcomes in carriers (Spearman r = 0.1912 and 0.1040, p = 0.1542 and 0.2584 respectively for carriers without FXTAS and with FXTAS), suggesting a shrinkage of the mitochondrial matrix (or the TCA cycle) with respect to the electron transport chain. This evidence concerns the gene CS and fragile X-associated tremor/ataxia syndrome.