Primary skin fibroblasts from FRDA patients are deficient in FXN and have been shown to provide a suitable model of the signature molecular characteristics of the disease.19 The cell lines were challenged with increasing doses of UVA ranging from 100 to 500 kJ m–2 (ca. 30 min to 2.6 h exposure to sunlight at sea level47) and the MTT assay was performed 24 h post-irradiation (Fig. 1a). Here, FXN is linked to Friedreich ataxia.