In an era of new therapies capable of targeting DNA and RNA, the known single genetic cause of Huntington's disease provides an attractive target for such treatments, with several huntingtin-lowering drugs now in human trials.1, 2 An appropriate time to initiate therapy in a preventive trial would be before clinical functioning has been affected, but when one or more measurable biomarkers of neurodegeneration can be used for enrichment or stratification and to monitor efficacy. This evidence concerns the gene HTT and juvenile Huntington disease.