Mediator, originally discovered in yeast and composed of multi-subunits with a high molecular mass , is an essential component of the RNA polymerase Ⅱ general transcriptional machinery and plays a crucial part in the activation and repression of eukaryotic mRNA synthesis. Regulatory information could be conveyed through changes in Mediator conformation that would influence the transcription initiation process. Recent studies have defined the subunit composition and associated activities of mammalian Mediator, and revealed a striking evolutionary conservation of Mediator structure and function from yeast to man.