A protein that forms part of the fibrous network (cytoskeleton) underlying the plasma membrane of erythrocytes and other cell types. Spectrins essentially consist of two helical polypeptide subunits that associate to form long flexible molecules. These are arranged in a polygonal mesh ‘glued’ together by short actin filaments and attached to the plasma membrane by ankyrin proteins. This mesh is involved in maintaining cell shape, particularly the biconcave shape of erythrocytes, and also serves as a platform for membrane channels, receptors, and transporters. Spectrins are also found in heart muscle cells closely associated with the contractile fibres.