FXN and hyperinsulinemic hypoglycemia, familial, 4: Another prominent feature of Fxn deficiency mouse and FRDA patients is iron accumulation and deficiency in activity of the iron-sulfur cluster dependent enzyme, aconitase, in cardiac muscle (Puccio et al., 2001; Rötig et al., 1997; Delatycki et al., 1999; Michael et al., 2006).