The DNA damage response (DDR) is a complex network that assists the completion and fidelity of DNA replication upon DNA insults. Because cancer cells are subject to high levels of replication stress in cancer cells, they heavily rely on the DDR. Conventional anticancer therapy exploits this vulnerability by inhibiting DDR effectors. Checkpoint Kinase 1 (Chk1) is a crucial mediator of the DDR whose inhibition is undergoing clinical evaluation, especially in prostate, ovarian and triple-negative breast cancers. It is currently accepted that DDR inhibitors trigger cell death as a consequence of increased replication stress and the ensuing chromosome instability (CIN). The link between replication stress and cell death has been validated in Chk1-deficient cell models; however, no unambiguous relationship has been established between replication stress, CIN, and cell death. Given that CIN fuels drug resistance, elucidating the molecular triggers of CIN and their relevance to cell survival is central to cancer research. We will present data, published in Calzetta et al., Sci. Adv., 2020, that unravel the identity of the molecular effectors of CIN activated by Chk1 deficiency. Unexpectedly, the pathway leading to CIN is independent of the one causing replication stress-dependent cell death. We propose that cancer treatment with Chk1 inhibitors might be improved by repressing the CIN pathway identified by us to avoid or reduce the generation of mutations that promote drug resistance.