Schematic of P-TEFb (composed of CycT and CDK9 subunits) mediated release of promoter-proximal paused Pol II into productive elongation. P-TEFb phosphorylates serine 2 of the Pol II carboxyl-terminal domain (CTD) to stimulate elongation. BRD4/fs(1)h binds to acetylated histones and helps recruit P-TEFb.
