What does IF stand for?

Intermediate Filament

Intermediate filament are cytoskeletal components found in metazoan cells. 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, and that of 25 nm microtubules, although they were initially designated 'intermediate' because their average diameter is between those of narrower microfilaments and wider myosin filaments found in muscle cells.