DB18 is a potent small-molecule inhibitor of CDC2-like kinases CLK1, CLK2, and CLK4, with IC₅₀ values in the 10–30 nM range. It demonstrates exceptional selectivity, showing no activity against the closely related DYRK1A kinase even at 100 µM. This compound is widely used in chemical biology research to dissect CLK signaling pathways and to study the structural basis of kinase selectivity through molecular simulation and structure-activity relationship analyses.