Western blot using affinity purified anti-CDK9 pT29 antibody shows detection of phosphorylated CDK9. 100 ng of purified P-TEFb, which contains CDK9 and its regulatory cyclin T1 subunit, was incubated with ATP in the presence or absence of Brd4, a protein known to induce CDK9 phosphorylation at T29. The primary antibody was used at a 1:1000 dilution. Personal Communication, J. Brady, NCI, Bethesda, MD.