NEFL and juvenile Huntington disease: To address these issues and to enable head-to-head comparison of NfL with other proposed biochemical markers, we have recruited 80 participants in whom NfL concentrations in CSF will be measured longitudinally, supported by neuroimaging,28 and have launched a multisite CSF study, HDClarity (NCT02855476), that will include 600 participants with premanifest to advanced Huntington's disease and controls.