sentences:  For reasons that are not well understood, this primarily affects the tissue of the dorsal root ganglia, cerebellum, and heart muscle.In mice, complete inactivation of the FXN gene is lethal in the early embryonic stage.Although nearly all organisms express a frataxin homologue, the GAA repeat in intron 1 only exists in humans and other primates, so the mutation that causes FDRA can't occur naturally in other animals.