FXN and Friedreich ataxia: Nuclear-encoded human frataxin is synthesized in the cytoplasm as a precursor polypeptide of 210 amino acids; subsequently, it is imported inside mitochondrion where it undergoes a two-step maturation by the mitochondrial processing peptidase (MMP) [9,10,11]: MMP first cleaves a portion of the N-terminus, the mitochondrial import signal, producing the intermediate form (FXN 42–210), and in the second step, MMP produces the mature form (FXN 81–210), which is the most abundant form found both in normal individuals and FRDA patients [9].