In contrast, the YG8s.300 and YG8s.800 mouse models of FRDA harbor a human FXN YAC transgene with ~300 and 800 GAA units, respectively, that undergo progressive instability starting at ~18 weeks of age that is biased toward expansion of repeats in somatic tissues, including the cortex, striatum and the liver, but not the tail110–112. This evidence concerns the gene FXN and Friedreich ataxia.