Resorufin - a lead for a new protein kinase CK2 inhibitor Screening of a natural compound library led to the identification of resorufin, as a highly selective and potent inhibitor for protein kinase CK2. Out of 52 kinases tested only CK2 was inhibited. The IC50 values determined for the CK2 holoenzymes were 1.5 µM and for the free catalytic subunits ca. 4 µM. In different cancer cell lines treatment with resorufin led to cell death and endogenous CK2 was inhibited.