RNA polymerase II promoter-proximal stalling and subsequent escape to transcriptional elongation. At many genes, RNA polymerase II (Pol II) stalls after the initiation of transcription, producing a short transcript typically less than 50 nucleotides long (left). Escape from stalling (right) is induced by developmental or environmental signals. In the stalled complex, only Ser5 of the carboxy-terminal domain (CTD) of Pol II is phosphorylated . The P-TEFb complex (composed of CDK9 and cyclin T) facilitates release of Pol II from stalling by phosphorylating DSIF, NELF and the carboxy-terminal domain of Pol II at Ser2 residues [8,11]. See text for details of other proteins shown in the diagram.