This expansion causes epigenetic changes and formation of heterochromatin near the repeat. The length of the shorter GAA repeat is correlated with the age of onset and disease severity. The formation of heterochromatin results in reduced transcription of the gene and low levels of frataxin. People with FDRA might have 5-35% of the frataxin protein compared to healthy individuals. Heterozygous carriers of the mutant FXN gene have 50% lower frataxin levels but this decrease is not enough to cause symptoms. In about 4% of cases the disease is caused by a (missense, nonsense, or intronic) point mutation, where the patient has an expansion in one allele and a point mutation in the other. A missense point mutation can have milder symptoms. Depending on the point mutation a patient may end up with no frataxin, nonfunctional frataxin or frataxin that is not properly localized to the mitochondria. FRDA affects the nervous system, heart and pancreas and other systems. Degeneration of nerve tissue in the spinal cord causes ataxia. The sensory neurons essential for directing muscle movement of the arms and legs through connections with the cerebellum are particularly affected. The disease primarily affects the spinal cord and peripheral nerves. The spinal cord becomes thinner and nerve cells lose some myelin sheath. The diameter of the spinal cord is smaller than that of unaffected individuals mainly due to smaller dorsal root ganglia. The motor neurons of the spinal cord are affected to a lesser extent than sensory neurons. In peripheral nerves there is a loss of large myelinated sensory fibers. Structures in the brain are also affected by FRDA, notably the dentate nucleus of the cerebellum. In the heart, FRDA patients often develop some fibrosis, and over time many patients develop left ventricle hypertrophy and dilatation of the left ventricle. The exact role of frataxin remains unclear. Frataxin assists iron-sulfur protein synthesis in the electron transport chain to generate adenosine triphosphate, the energy molecule necessary to carry out metabolic functions in cells. Frataxin also regulates iron transfer in the mitochondria by providing a proper amount of reactive oxygen species (ROS) to maintain normal processes. One result of frataxin deficiency is mitochondrial iron overload, which damages many proteins due to effects on cellular metabolism. Without frataxin, the energy in the mitochondria falls, and excess iron creates extra ROS, leading to further cell damage.