Intermediate filaments (IFs) are cytoskeletal components found in the cells of many animal species. They are composed of a family of related proteins sharing common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin (microfilaments), and that of 25 nm microtubules, and they were initially designated 'intermediate' because their average diameter is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells. Most types of intermediate filaments are cytoplasmic, but one type, the lamins, are nuclear.