SEL120-34A monohydrochloride is an ATP-competitive and selective CDK8 inhibitor, inhibits kinase activities of CDK8/CycC and CDK19/CycC complexes with IC50s of 4.4 nM and 10.4 nM, respectively, with a Kd of 3 nM for CDK8. SEL120-34A monohydrochloride weakly inhibits CDK9 (calculated IC50=1070 nM), but shows no obvious activity against CDK1, 2, 4, 6, 5, 7. SEL120-34A monohydrochloride inhibits phosphorylation of STAT1 S727 and STAT5 S726. Has anti-tumor activity.