DNA replication is fundamental to life but also underpins diseases such as cancer. The identification of molecular pathways that differentiate cancer cells from healthy ones, has led to the discovery of critical therapeutic tools that selectively impair DNA replication in cancer cells. Our work has focused on the identification of factors that impair mechanisms of adaptation of DNA replication to the damaged DNA templates that accumulate after chemotherapy. Our research has revealed that a small peptide corresponding to the C terminus of the CDK inhibitor p21 is a versatile enhancer of the cytotoxicity cause by a variety of chemotherapeutic agents. We have also identified novel enhancers of cytotoxicity in cells deficient in homologous recombination repair. Recently, we have investigated alternative tumor targets that do not involve cell killing, focusing on chromosomal instability (CIN). Our findings suggest that CIN can be mitigated under certain chemotherapeutic conditions without reducing the treatment’s cytotoxic effects. This property could potentially be exploited to improve cancer therapies by preventing tumor adaptation to treatment, a major cause of therapeutic failure. These insights provide a framework for developing novel strategies that combine chemotherapy with mechanisms to stabilize chromosomes, offering promising avenues for enhancing cancer treatment outcomes.