In several contexts they provide quantitative measures of brain pathology: this includes providing a therapeutic biomarker for a central nervous system pathology (NfL in multiple sclerosis [19]), and a marker of pre-symptomatic neurodegenerative disease activity (NfL in carriers of mutations causing familial Alzheimer’s [20] and Huntington’s diseases [21]). The gene discussed is NEFL; the disease is juvenile Huntington disease.