Mediator complex subunit 15 is a 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. MED15 is required for cholesterol-dependent gene regulation. It also positively regulates the Nodal signaling pathway. [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/Q96RN5].