The most potent inhibitor,CAN508, reduced the frequency of S-phase cells of the cancer cell line HT-29 in antiproliferation assays.?Further observed cellular effects included decreased phosphorylation of the retinoblastoma protein and the C-terminal domain of RNA polymerase II, inhibition of mRNA synthesis, and induction of the tumor suppressor protein p53, all of which are consistent with inhibition of CDK9.