Structural domains comprising this superfamily share the structure of subunit 6 of the mediator complex, a co-activator involved in the regulated transcription of nearly all RNA polymerase II-dependent genes. Mediator connects transcriptional regulators and Pol II. This mediator complex is comprised of several subunits, the number of these varies between organisms, which form the head, middle, tail and kinase modules. The cited subunit 6 is a conserved subunit of the head module. The head module interacts with Pol II and this contact is required for regulated transcription PMID:23123849.