Selective inhibitor of polo-like kinase 1 (PLK1) and polo-like kinase 3 (PLK3) (IC50 values are 2.2 and 9.1 nM respectively). Displays > 100-fold selectivity over ~30 other kinases tested including cdk1 and cdk2. Inhibits proliferation of most tumor cells in vitro and is selective over normal diploid fibroblasts.