Neurofilament triplet H protein (NF-H) is one of the three intermediate filament proteins (L, M and H) making up neurofilaments. NF-H is involved in the maintenance of neuronal caliber. NF-H has an important function in mature axons that is not subserved by the two smaller NF proteins. Phosphorylation plays a major role in the functioning of the larger neurofilament polypeptides (NF-M and NF-H), the levels of phosphorylation being altered developmentally and coincident wih a change in the neurofilament function. Defects in NF-H are a cause of susceptibility to amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder affecting upper and lower motor neurons resulting in fatal paralysis.