Neurofilament L (NF-L) is a protein subunit that is a component of neurofilaments, which are intermediate filaments found in neurons. Neurofilaments are major cytoskeletal elements in neuronal axons and dendrites and play an important role in maintaining the structural integrity and function of neurons. NF-L, along with two other protein subunits (NF-M and NF-H), forms the neurofilament triplet, which is the basic building block of neurofilaments. NF-L is the lightest subunit and is encoded by the NEFL gene. In recent years, NF-L has been found to be a useful biomarker for assessing neuronal injury and degeneration in various neurological diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), multiple sclerosis, and Huntington's disease. When neurons or axons are damaged or degenerate, NF-L is released into the cerebrospinal fluid (CSF) or blood, and its levels in these fluids can be measured to assess the extent of neuronal injury.