GFAP and early-onset autosomal dominant Alzheimer disease: Neurofilament light chain (NfL) is a neuroaxonal skeletal polypeptide elevated in several stable neurodegenerative diseases, indicating neuronal cell damage and eventual death.9,10 Glial fibrillary acidic protein (GFAP), an astrocyte cytoskeletal filament protein marker of astrogliosis and astrocyte degeneration, is elevated in neurodegeneration and correlated to Alzheimer’s disease severity.11