NEFL and amyotrophic lateral sclerosis: However, NfL lacks disease specificity, as elevated levels are also observed with normal aging and across a range of neurodegenerative disorders, including multiple sclerosis (MS), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), multiple system atrophy (MSA), and Parkinson’s disease (PD).