Spectrins are central components of the cytoskeleton that form a scaffold below the plasma membrane. Spectrins contain two subunits, alpha and beta, which intertwine to form heterodimers that can self associate into elongated tetramers. alpha-spectrin I and beta-spectrin I form heterodimers in red blood cells, while nonerythroid mammalian cells contain heterodimers of alpha-spectrin I and II with beta-spectrin I to V. The structure of spectrins includes a succession of triple-helical repeats along with various domains, such as SH3 domain, EF hands, PH domains, and binding domains for ankyrin, actin, band 4.1, and calmodulin. alpha-spectrin II is a widely expressed non-erythroid spectrin that contains an SH3 domain, a calmodulin binding site, and two cleavage sites, one at Tyr-1176 for calpains and one at Asp-1185 for caspase-3. alpha-spectrin II and beta-spectrin II, like many other spectrins, can form heterodimers that can self associate into tetramers, as well as interact with Band 4.1, F-actin, and other proteins near the plasma membrane.