CDK1 and leukemia: In addition to serine 506, we found serine 112 was phosphorylated in CK2-treated R-topo I. This site has previously been identified as a cdk-1 target in topo I from A549 human lung cancer cells and K562 human leukemia cells [14], although in that study the phosphorylation status of serine 112 was not found to affect topo I activity.