Structural domains comprising this superfamily share the structure of the N-terminal domain of subunit B of DNA polymerase alpha-primase, an enzyme which is essential for DNA replication in higher eukaryotes as it initiates synthesis on both leading and lagging strand single-stranded DNA templates. It consists of a primase heterodimer that synthesizes RNA primers, a DNA polymerase that extends them, and a fourth subunit, p68 (also termed B-subunit), which is believed to regulate the whole complex. This domain docks on the SV40 helicase and activates the viral primosome PMID:20234039.