Within humans, there are only six actin isoforms, and these isoforms are responsible for an array of unique and complex cellular structures and key cellular interactions. It is thought that the function and form of the actin cytoskeleton is controlled largely by actin-binding proteins (ABP) that are associated with the actin polymer. ABP are a group of proteins that bind to actin. Although tropomyosin is sometimes included as an ABP, it is not a true ABP. The tropomyosin dimer has very low affinity for an actin filament and forms no van der waals contacts with actin. It is only the formation of a tropomyosin polymer's winding around the actin filament that provides stability to the tropomyosin-actin filament interaction.