Potent, ATP-competitive inhibitor of Clk-family kinases (IC50 values are 20, 200 and 15 nM for mClk1, 2 and 4 respectively and >10 for mClk3). Suppresses serine/arginine-rich protein phosphorylation; affects the regulation of alternative splicing by phosphorylation of SR protein both in vivo and in vitro.