The spectrin gene family encodes a diverse group of cytoskeletal proteins that include spectrins, dystrophins and ?-actinins. There are four tissue-specific ?-actinins, namely ?-actinin-1, ?-actinin-2, ?-actinin-3 and ?-actinin-4, which are localized to muscle and non-muscle cells, including skeletal, cardiac and smooth muscle cells, as well as within the cytoskeleton. Each ?-actinin protein contains one Actin-binding domain, two calponin-homology domains, two EF-hand domains and four spectrin repeats, through which they function as bundling proteins that can cross-link F-Actin, thus anchoring Actin to a variety of intracellular structures. Defects in the gene encoding ?-actinin-4 are the cause of focal segmental glomerulosclerosis 1 (FSGS1), a common renal lesion characterized by decreasing kidney function and, ultimately, renal failure.