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intermediate filament

n. (context protein English) Any of a family of twisted protein filaments intermediate in size between microfilaments and microtubules

Wikipedia
Intermediate filament

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. Initially designated 'intermediate' because their average diameter (10 nm) is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells, the diameter of Intermediate filaments is now commonly compared to actin microfilaments (7 nm) and microtubules (25 nm). Most types of intermediate filaments are cytoplasmic, but one type, the lamins, are nuclear.