NEFL and familial amyotrophic lateral sclerosis: The behavior of NfL levels over time can vary between neurodegenerative diseases,17, 37, 38, 39 but usually they increase or remain constant over time and with disease manifestation, as shown in, for example, genetic frontotemporal dementia,39, 40 sporadic Creutzfeldt‐Jakob disease,41 Charcot‐Marie‐Tooth disease,42 Huntington's disease,17 familial amyotrophic lateral sclerosis,43 and autosomal dominant Alzheimer's disease,44 as well as other genetic ataxias such as RFC1 disease45 and certain spinocerebellar ataxias.32, 33, 34