Neurofilaments are the 10 nanometer (10nm) intermediate filaments found specifically in neurons. They are a major component of the cell's cytoskeleton, and provide support for normal axonal radial growth (i.e. increases in axon's diameter). Neurofilaments are composed of polypeptide chains or subunits that are related structurally to the intermediate filaments of other tissues such as keratin subunits, which make 10nm filaments expressed specifically in epithelia. The family of proteins making intermediate filaments is divided into 5 major classes, the keratins forming the classes I and II. Class III contains the proteins Vimentin, Desmin, Peripherin and Glial Fibrillary Acidic Protein (GFAP). The major neurofilament subunits occupy the class IV family of intermediate filaments. Finally class V contains the nuclear lamins.