Concentrations in CSF are increased in people with neurodegenerative diseases, including Alzheimer's disease,13 amyotrophic lateral sclerosis,13 and frontotemporal dementia.13, 14 Four small-scale cross-sectional studies found raised concentrations of NfL in the CSF of individuals with Huntington's disease,8, 15, 16, 17 and we have shown a close association between increased concentrations of NfL and mHTT in CSF.8 The gene discussed is NEFL; the disease is early-onset autosomal dominant Alzheimer disease.