CLK1 4-IN-1 (compound 31) is a highly potent and selective inhibitor of Clk1 and Clk4, with IC50 values of 9.7 nM and 6.6 nM, respectively. It effectively inhibits the growth of T24 cancer cells, with a GI50 value of 1.1 μM. CLK1 4-IN-1 has shown promising potential as an anticancer research tool [1].