RNA polymerase II (RNAP II and Pol II) is a multiprotein complex that transcribes DNA into precursors of messenger RNA (mRNA) and most small nuclear RNA (snRNA) and microRNA. The purified enzyme has typically 10–12 subunits (12 in humans and yeast). The subunit RPB1 contains a carboxy terminal domain (CTD) composed of up to 52 heptapeptide repeats (YSPTSPS) that are essential for polymerase activity.