Cyclin-dependent kinases (CDKs) play a key role in regulating cell division by phosphorylating distinct substrates in different phases of the cell cycle. Cell cycle deregulation in many cancers often results from altered CDK activity. Thus, CDKs are potential pharmacological targets for anticancer agents. CAY10574 is a potent, selective inhibitor of Cdk9 in vitro with an IC50 value of 0.35 µM. It is also a competitive inhibitor of Cdk2-cyclin E with respect to ATP, with Ki and IC50 values of 13.3 and 20 µM, respectively. CAY10574 reduces the population of S-phase cells of the cancer cell line HT-29 and blocks proliferation of several other cancer cell lines including MCF-7, HOS, G361, and K562 cells with IC50 values of 33, 49, 64, and 62 µM, respectively.