The activation of gene transcription is a multistep process that is triggered by factors that recognize transcriptional enhancer sites in DNA . These factors work with co-activators to direct transcriptional initiation by the RNA polymerase II apparatus. The mediator of RNA polymerase II transcription subunit 1 protein is a subunit of the CRSP (cofactor required for SP1 activation) complex, which, along with TFIID , is required for efficient activation by SP1. This protein is also a component of other multisubunit complexes [., thyroid hormone receptor -(TR-) associated proteins that interact with TR and facilitate TR function on DNA templates in conjunction with initiation factors and cofactors]. It also regulates p53 -dependent apoptosis and it is essential for adipogenesis . This protein is known to have the ability to self-oligomerize. 
Using transient transfection assays, Fan et al. (1999) demonstrated that BRCA1 inhibits signaling by the ligand-activated estrogen receptor ESR-alpha through the estrogen-responsive enhancer element and blocks the C-terminal transcriptional activation function AF2 of ESR-alpha. These results suggested that wildtype BRCA1 protein may function, in part, to suppress estrogen-dependent mammary epithelial proliferation by inhibiting ESR-alpha-mediated transcriptional pathways related to cell proliferation, and that loss of this ability may contribute to tumorigenesis.