[Regulation of mRNA synthesis requires intermediary proteins that transduce regulatory signals from upstream transcriptional activator proteins to basal transcription machinery at the core promoter. Mediator Complex Component Genes encode the mediator complex, one type of intermediary and which consists of approximately 20 polypeptides. It is physically associated with RNA polymerase II and is essential for transcriptional activation. (from OMIM 602984) ( NCI )]