The majority of FRDA patients are homozygous for large expansions of GAA repeat sequences in intron 1 of the FXN gene, whereas a small fraction of patients are compound heterozygotes with an expanded GAA repeat sequence in one FXN allele and a missense or nonsense mutation in the other (Cossée et al., 1999). This evidence concerns the gene FXN and Friedreich ataxia.