Neurofilament light (NfL) is a recognized biomarker of subcortical large-caliber axonal degeneration.1,2 CSF and plasma NfL levels are elevated in multiple neurodegenerative disorders, including Alzheimer disease (AD) dementia,3,4 frontotemporal dementia,5 multiple sclerosis,6 and traumatic brain injury.7,8 Although the NfL elevation is lower in AD dementia compared to other neurodegenerative disorders (e.g., vascular dementia and frontotemporal dementia),9,10 NfL is hypothesized to be a nonspecific marker of neurodegeneration. The gene discussed is NEFL; the disease is early-onset autosomal dominant Alzheimer disease.