Intermediate Filaments (IFs) are neuronal cytoskeletal structures that are at least partly composed of the neurofilament (NF) triplet proteins, the light (NF-L), mid-sized (NF-M), and heavy (NF-H) neurofilament subunits. These proteins are encoded by separate genes and migrate at ~ 68, 150, and 200 kDa, respectively, on SDS-PAGE. NF proteins have an unusually high content of phosphoserine residues, located in lysine-serine-proline (KSP) repeat regions; NF-H subunits may contain as many as 40-50 repeats in tandem arrays; NF-M subunits contain variable numbers of KSP repeats.