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. {ECO:0000269|PubMed:12167862, ECO:0000269|PubMed:16630888, ECO:0000269|PubMed:16799563}. This is the function of MED15 (mediator complex subunit 15, Ensembl gene identifier ENSG00000099917).