Recent studies highlight the complementaryrole of genetic approachesin preclinical drug characterization., Here, we combinedgenetic tools and chemical probes to first assess the selectivityof our tool compounds and subsequently the roles of Mediator kinasesin colon cancer models.−,  We hypothesized that on-targeteffects of our small-molecule inhibitors would be evident in parentcell lines but absent in CDK8/CDK19 knockouts, while off-target effectswould impact both parent cells and knockout clones equally. Here, CDK8 is linked to colonic neoplasm.