<p>This domain consists of a multi-helical fold comprised of two curved layers of alpha helices arranged in a regular right-handed superhelix, where the repeats that make up this structure are arranged about a common axis [<cite idref="PUB00015442"/>]. These superhelical structures present an extensive solvent-accessible surface that is well suited to binding large substrates such as proteins and nucleic acids. This topology has been found with a number of repeats and domains, including the armadillo repeat (found in beta-catenins and importins), the HEAT repeat (found in protein phosphatase 2a and initiation factor eIF4G), the PHAT domain (found in Smaug RNA-binding protein), the leucine-rich repeat variant, the Pumilo repeat, and in the H regulatory subunit of V-type ATPases. The sequence similarity among these different repeats or domains is low, however they exhibit considerable structural similarity. Furthermore, the number of repeats present in the superhelical structure can vary between orthologues, indicating that rapid loss/gain of repeats has occurred frequently in evolution. A common phylogenetic origin has been proposed for the armadillo and HEAT repeats [<cite idref="PUB00015443"/>].</p>