FXN and Mitochondrial myopathy: In this regard, it is noteworthy that several mouse models of mitochondrial myopathy, as well as the KIKO model of FRDA, show a progression in phenotype despite stable residual expression of FXN or the other protein being studied; this appears to be the case when the protein is depleted in early life (8, 34, 35, 38, 53) and in adulthood (34, 35).