Abstract Normal cells employ several fundamental brake mechanisms to tightly coordinate the execution of different cell cycle phases. Loss of these checkpoint controls is a hallmark of cancer. Our goal is to devise therapeutic strategies targeting newly discovered checkpoint perturbation mechanisms in cancers harboring mutant p53. Tumor suppressor p53 is a key regulator for G1 cell cycle checkpoint. Mutant p53 mediates gain of functions to promote tumor progression and drug resistance. Nevertheless, how to target mutant p53 in cancer remains a big challenge. Our preliminary study identifies several novel mechanisms by which mutant p53 interacts with a checkpoint activator protein TopBP1 physically and functionally to perturb the checkpoint control, and induces ACTL6A to promote cisplatin resistance. We propose that the mutant p53-TopBP1 axis is a valid therapeutic target in cancer. We have developed two classes of novel TopBP1 inhibitors that can target the mutant p53- TopBP1 axis. In this proposal, we will determine their therapeutic potential in targeting checkpoint dysfunction in cancer. First, we will study how the mutant p53-TopBP1 axis perturbs replication control. Second, we will determine the functional interaction between mutant p53 and ACTL6A in growth control and cisplatin resistance. Third, we will investigate new combinations of targeted therapy against breast and ovarian cancers. The novel TopBP1 inhibitors that we have developed will greatly enhance the feasibility and impact of this proposal. In short, the proposed study will elucidate novel mechanisms of checkpoint perturbation and translate our discoveries into new therapeutics for a broad range of cancers.