Potent and selective CLK inhibitor (IC50 values are 28, 88 and 510 nM for CLK4, 1 and 2, respectively). Exhibits no significant activity at CLK3, DYRK, CDK or GSK3 at a concentration of 10 μM. Inhibits growth of a range of cancer cell lines in vitro at subnanomolar concentrations. Purity 98%. MW (observed): 282.72