NEFL and Ataxia: To compare preataxic SCA3 subjects with controls at the same age, we expressed the measured NfL level of SCA3 subjects as NfL z‐score in relation to the age‐dependent NfL distribution in controls (Fig 3B), and analysed the NfL z‐score as a function of the time to the estimated onset of ataxia (Fig 3C).