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. Essential for embryogenesis, including development of the central nervous system, heart, liver and placenta and for erythropoiesis. Also required for normal transcriptional control of thyroid-stimulating hormone beta (TSHB) in the pituitary. Acts as a coactivator for GATA1-mediated transcriptional activation during erythroid differentiation of K562 erythroleukemia cells (By similarity). Component of the Mediator complex, which is composed of MED1, MED4, MED6, MED7, MED8, MED9, MED10, MED11, MED12, MED13, MED13L, MED14, MED15, MED16, MED17, MED18, MED19, MED20, MED21, MED22, MED23, MED24, MED25, MED26, MED27, MED29, MED30, MED31, CCNC, CDK8 and CDC2L6/CDK11. The MED12, MED13, CCNC and CDK8 subunits form a distinct module termed the CDK8 module. Mediator containing the CDK8 module is less active than Mediator lacking this module in supporting transcriptional activation. Individual preparations of the Mediator complex lacking one or more distinct subunits have been variously termed ARC, CRSP, DRIP, PC2, SMCC and TRAP. This subunit specifically interacts with a number of nuclear receptors in a ligand-dependent fashion including AR, ESR1, ESR2, PPARA, PPARG, RORA, RXRA, RXRG, THRA, THRB and VDR. Interacts with CTNNB1, GABPA, GLI3, PPARGC1A and TP53. Interacts with GATA1 and YWHAH. Interacts with CLOCK; this interaction requires the presence of THRAP3. Interacts with CCAR1 (By similarity). Interacts with NR4A3 (PubMed:12709428). Interacts (via IBM motif) with PSIP1 (via IBD domain); phosphorylation increases its affinity for PSIP1 (By similarity). A subset of the protein may enter the nucleolus subsequent to phosphorylation by MAPK1 or MAPK3. Widely expressed in the adult, with high levels of expression in the liver, lung, intestinal mucosa, kidney cortex, thymic cortex, splenic follicle and seminiferous epithelium in testis. Also expressed in the adult heart, brain, spleen and skeletal muscle. Widely expressed during embryonic development; at stages 9.5 dpc-10.5 dpc, expression is strongest in neural tissues. At 11.5 dpc-12.5 dpc, expression is abundant throughout embryonic tissues, being strongest in the developing liver, primitive gut, nasopharynx, and developing limb buds. Moderately expressed at this stage in the brain and optic stalk, branchial arch and urogential ridge. Expressed at a low level in the heart. By stage 13.5 dpc-14.5 dpc, expression is abundant in the forebrain, vagus nerve, dorsal root ganglia, nasopharynx, kidney, liver, pancreas, intestine, gut, thymus, lung, genital tubercle, tongue and lower jaw. Moderately expressed in the midbrain and expressed at a low level in the heart and large blood vessels. In the developing placenta, expression is moderate in the giant and spongiotrophoblast cell layers and strongest in the labyrinthine portion throughout 9.5 dpc-13.5 dpc. Phosphorylated by MAPK1 or MAPK3 during G2/M phase which may enhance protein stability and promote entry into the nucleolus (By similarity). Phosphorylation increases its interaction with PSIP1 (By similarity). Belongs to the Mediator complex subunit 1 family. Truncated N-terminus.