Babcock et al. (1997) noted that post-mitotic, heavily aerobic cardiac muscle cells and neurons are heavily targeted in human disease. Thus, FRDA is considered a disease of both altered iron metabolism and respiratory deficiency. However, these phenotypes alone do not explain why other highly aerobic tissues and motor neuron types are spared in disease. FXN is most heavily studied in cardiac skeletal muscle, neurons, and fibroblasts for in vitro examination of cell disease profiles. This evidence concerns the gene FXN and Friedreich ataxia.