Spectrin repeat-domains are found in several proteins involved in cytoskeletal structure. These include spectrin, alpha-actinin and dystrophin. The sequence repeat used in this family is taken from the structural repeat in reference. The spectrin domain- repeat forms a three helix bundle. The second helix is interrupted by proline in some sequences. The repeats are defined by a characteristic tryptophan (W) residue at position 17 in helix A and a leucine (L) at 2 residues from the carboxyl end of helix C. Although the domain occurs in multiple repeats along sequences, the domains are actually stable on their own - ie they act, biophysically, like domains rather than repeats that along function when aggregated.