GPAA1 and Friedreich ataxia: Similar to the western blot results, both frataxin-E and frataxin-M were found to be deficient in FRDA patients with the LC–MS assay (n = 32 vs. n = 11 non-FRDA controls; Fig. 3A and Fig. S4A), and the deficiency was significantly correlated with the length of GAA1, which accounted for approximately half the variability (Fig. 3B and Fig. S4B).