Western blot using affinity purified CDK9 pT29 shows detection of phosphorylated CDK9. 100ng 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. 168093 was used at a 1:1000 dilution.