Mediator of RNA polymerase II transcription subunit 15, also known as Gal11,Spt13 in yeast and PCQAP, ARC105, or TIG-1 in humans is a protein encoded by the MED15 gene.

The protein encoded by this gene is a subunit of the multiprotein complexes PC2 and ARC/DRIP and may function as a transcriptional coactivator in RNA polymerase II transcription. This gene contains stretches of trinucleotide repeats and is located in the chromosome 22 region which is deleted in DiGeorge syndrome. Two transcript variants encoding different isoforms have been found for this gene. [provided by RefSeq, Jul 2008].
Function: Component of the Mediator complex, a coactivator involved in the regulated transcription of nearly all RNA polymerase II-dependent genes. Mediator functions as a bridge to convey information from gene-specific regulatory proteins to the basal RNA polymerase II transcription machinery. Mediator is recruited to promoters by direct interactions with regulatory proteins and serves as a scaffold for the assembly of a functional preinitiation complex with RNA polymerase II and the general transcription factors. Required for cholesterol-dependent gene regulation. Positively regulates the Nodal signaling pathway.