CHIR-124 is a quinolone-based small molecule that is structurally unrelated to other known inhibitors of Chk1. CHIR-124 interacts synergistically with topoisomerase poisons (e.g., Camptothecin or SN-38) in causing growth inhibition in a variety of cancer cell lines, including breast carcinoma (MDA-MB-231 and MDA-MB-435) and colon carcinoma (SW-620 and Colo205), all of which contains the mutant p53 gene.