FXN and Friedreich ataxia: For this reason, FA is commonly included among the nucleotide repeats expansion disorders and, although aberrant RNA structures and toxic gain of function of the RNA or protein products are believed to be the pathogenic mechanism characterizing many of this heterogeneous group of pathologies [22], the lack of the mitochondrial protein frataxin, the product of FXN [17], represents the molecular determinant underlying the disease.