Med21 has been known as Srb7 in yeasts, hSrb7 in humans and Trap 19 in Drosophila. The heterodimer of the two subunits Med7 and Med21 appears to act as a hinge between the middle and the tail regions of Mediator [2].

1: A unified nomenclature for protein subunits of mediator complexes linking transcriptional regulators to RNA polymerase II. Bourbon HM, Aguilera A, Ansari AZ, Asturias FJ, Berk AJ, Bjorklund S, Blackwell TK, Borggrefe T, Carey M, Carlson M, Conaway JW, Conaway RC, Emmons SW, Fondell JD, Freedman LP, Fukasawa T, Gustafsson CM, Han M, He X, Herman PK, Hinnebusch AG, Holmberg S, Mol Cell. 2004;14:553-557. PMID:15175151

2: A conserved mediator hinge revealed in the structure of the MED7.MED21 (Med7.Srb7) heterodimer. Baumli S, Hoeppner S, Cramer P; J Biol Chem. 2005;280:18171-18178. PMID:15710619