Cellular dysfunction due to FXN deficiency is presumed to be the result of a mitochondrial defect, since FXN localizes to mitochondria (Tan et al., 2001; Koutnikova et al., 1997; Foury and Cazzalini, 1997) and deficiencies of mitochondrial enzymes and function have been observed in tissues of FRDA patients (Rötig et al., 1997; Lodi et al., 1999). The gene discussed is FXN; the disease is Friedreich ataxia.