As a class of cytoskeletal structures, intermediate filaments (IFs) display an unusual degree of cell specificity and are often used as markers of cellular differentiation. They comprise a family of related genes that have been classified in five types. All five share homology in a core rod domain, which contains multiple α-helical domains that can form coiled coils. The sequence homology in this conserved domain is sufficient that some antibodies recognize all known IFs from mammals through a wide range of invertebrates. IFs are also ultrastructurally similar regardless of type, forming 8- to 10-nm rope-like filaments that may be several micrometers long. However, NFs differ from the other IFs because they have sidearms that project from the surface (Fig. 8-2). The result is that IFs in non-neuronal cells are often seen in densely packed bundles, while NFs are widely spaced (Fig. 8-4).