SEL120-34A HCl is a selective, ATP-competitive CDK8 inhibitor, with IC50 of 4.4 nM and 10.4 nM for CDK8/CycC and CDK19/CycC, respectively. SEL120-34A HCl shows no obvious inhibition on CDK1, 2, 4, 6, 5, 7, and only weakly suppresses CDK9 (IC50, 1070 nM). SEL120-34A HCl is active against a panel of AML cell lines (GI50<1 μM), such as SKNO-1, KG-1, HEL-60, MOLM-16, MV-4-11, OciAML-2, MOLM-6 and OciAML-3 cells, consistent with the effective inhibition range of STAT1 S727 and STAT5 S726.