RNA polymerase II subunit B1 (RPB1) is the largest subunit of the RNA polymerase II complex. As a holoenzyme RNA polymerase II catalyzes transcription of eukaryotic DNA into RNA using the four ribonucleoside triphosphates as substrates. The RBP1 subunit, in combination with other polymerase subunits, forms a large central cleft that maintains contact between the active site of the enzyme, the DNA template, and the nascent RNA transcript. This subunit also contains a carboxy terminal domain (CTD) consisting of tandem heptapeptide repeats. Phosphorylation activates the RNA polymerase II beta subunit, allowing it to serve as an assembly platform for additional subunits that modulate initiation, elongation, termination and mRNA processing. In actively transcribing RNA polymerase ‘Ser-2’ and ‘Ser-5’ of the heptapeptide repeat are phosphorylated. Ser-7 is phosphorylated before initiation of transcription at promoter regions.