NfL has been proposed as a potential surrogate endpoint indicating neuroprotection for multiple neurological disorders (e.g., MS [42], ALS [43], and HD [7]); its utility as a safety biomarker for neurotoxicity is also being considered [44, 45]; and it may be further utilised for enrichment and stratification of presymptomatic populations to facilitate prevention trial design [46]. The gene discussed is NEFL; the disease is Huntington disease.