In mammalian cells, transcription is regulated in part by high molecular weight co-activating complexes that mediate signals between transcriptional activators and RNA polymerase II (Pol II). The mediator complex is one such multi-protein structure that functions as a bridge between regulatory proteins and Pol II, thereby regulating Pol II-dependent transcription. Med18 (mediator complex subunit 18), also known asp28b, is a 208 amino acid protein that localizes to nucleus and exists as a component of the mediator complex. Working in tandem with several other proteins, including Med8 and Med25, Med18 serves as a scaffold for the assembly of a functional preinitiation complex with Pol II and general transcription factors, thereby activating the transcription of Pol II-dependent genes.