Topoisomerase I (topo I) is required to unwind DNA during synthesis, and provides the unique target for camptothecin-derived chemotherapeutic agents, including Irinotecan and Topotecan. While these agents are highly effective anticancer agents, some tumors do not respond due to intrinsic or acquired resistance, a process that remains poorly understood. Because of treatment toxicity, there is interest in identifying cellular factors that regulate tumor sensitivity and might serve as predictive biomarkers of therapy sensitivity. Here we identify the serine kinase, protein kinase CK2, as a central regulator of topo I hyperphosphorylation and activity, and cellular sensitivity to camptothecin. In 9 cancer cell lines and 3 normal tissue-derived cell lines we observe a consistent correlation between CK2 levels and camptothecin responsiveness. Two other topo Itargeted serine kinases, protein kinase C and cyclin-dependent kinase1, do not show this correlation. Camptothecin-sensitive cancer cell lines display high CK2 activity, hyperphosphorylation of topo I, elevated topo I activity, and elevated phosphorylation-dependent complex formation between topo I and p14ARF, a topo I activator. Camptothecin-resistant cancer cell lines and normal cell lines display lower CK2 activity, lower topo I phosphorylation, lower topo I activity, and undetectable topo I/p14ARF complex formation. Experimental inhibition or activation of CK2 demonstrates that CK2 is necessary and sufficient for regulating these topo I properties and altering cellular responses to camptothecin. The results establish a cause and effect relationship between CK2 activity and camptothecin sensitivity, and suggest that CK2, topo I phosphorylation, or topo I/p14ARF complex formation could provide biomarkers of therapy responsive tumors. Figure S1), (2) Effect of CK2 activator on purified PKC, cdk1, and CK2 activities, and on endogenous PKC and cdk1 activity in H23 cells ( Figure S2), (3) Effect of CK2 activator on CK2α transcription ( Figure S3), (4) Levels of serine/threonine phosphatase activity in cell lines ( Figure S4), and (5) Topo-I phosphorylation levels in A549 and K562 cell lines ( Figure S5). The material can be accessed free of charge via the Internet at