To investigate the physiological role of TopBP1 in the checkpoint response in cancer cells expressing high levels of TopBP1, we next performed BrdU incorporation assay to examine how a modest (less than 50%) or high (more than 90%) degree of TopBP1 depletion may affect DNA synthesis after HU treatment in TopBP1-depleted MDA-MB468 stable cells. This evidence concerns the gene TOPBP1 and cancer.