Inhibition effects of representative natural compounds on CK2 kinase activity. A. Natural compounds from NCI plates were screened for their CK2 inhibition activities at 10 μM using a kinase assay. Relative kinase activity of CK2 is shown on the Y axis as a relative number to the control that did not have inhibitor treatment. Compounds 1-F4 (coumestrol), 1-D6 (curcumin) and 1-F3 (aristolochic acid I) showed more than 50% inhibition on kinase activity at 10 μM. Data represents the average of duplicates and bars indicate standard deviation. 1-X represents compounds from NCI plate 13091250 and 2-X represents compounds from NCI plate 13091251. B. Chemical structures of coumestrol, curcumin and aristolochic acid I. C. IC50 of coumestrol.