The Mediator complex is a co-activator involved in the regulated transcription of almost all RNA polymerase II-dependent genes. It remains bound to the promoter when transcriptional elongation begins, following the dissociation from the RNA polymerase II holoenzyme. The Mediator complex is composed of several subunits subunits and structural domains comprising this superfamily share the structure of subunit Med21. The different subunits form at least three structurally distinct submodules -head, middle and tail. The head and the middle modules interact directly with RNA polymerase II, whereas the elongated tail module interacts with gene-specific regulatory proteins. Med21 interacts with Med7 to form a heterodimer which is a highly conserved part of the mediator. This heterodimer has been shown to binds to other highly conserved Mediator subunits via its evolutionarily conserved regions and therefore plays a central architectural role within the Mediator PMID:15710619.

Med21 has been known as Srb7 in yeasts, hSrb7 in humans and Trap 19 in Drosophila PMID:15710619.