Structural domains comprising this superfamily share the structure of the insert domain in RNA 3'-terminal phosphate cyclase. RNA 3'-terminal phosphate cyclase is a type of RNA cyclase which catalyses the conversion of 3'-phosphate to a 2',3'-cyclic phosphodiester at the end of RNA. It is comprised of two domains and its smaller insert domain disrupts the large domain.

These enzymes might be responsible for production of the cyclic phosphate RNA ends that are known to be required by many RNA ligases in both prokaryotes and eukaryotes PMID:10673421. The role of this insert domain remains unknown