Neurofilaments (NF) are classed as type IV intermediate filaments found in the cytoplasm of neurons. They are protein polymers measuring 10 nm in diameter and many micrometers in length. Together with microtubules (~25 nm) and microfilaments (7 nm), they form the neuronal cytoskeleton. The protein composition of neurofilaments varies widely across different animal phyla. Mammalian neurofilaments are heteropolymers of up to five different proteins: NF-L, NF-M, NF-H, α-internexin and peripherin. The precise composition of neurofilaments in any given nerve cell depends on the relative expression levels of the neurofilament proteins in the cell at that time. For example, NF-H expression is low in developing neurons and increases postnatally in neurons with myelinated axons.[6] In the adult nervous system neurofilaments in small unmyelinated axons contain more peripherin and less NF-H whereas neurofilaments in large myelinated axons contain more NF-H and less peripherin.