EXO1-IN-1 (Compound F684) is a potent and selective inhibitor of exonuclease 1 (EXO1) with an IC50 of 15.7 μM. It inhibits DNA end resection, promotes the accumulation of DNA double-strand breaks, and induces S-phase polyadenylation, thereby disrupting DNA repair pathways in homologous recombination-deficient (HRD) cancer cells. This selectivity effectively targets and kills tumor cells with homologous recombination gene defects, such as BRCA1/2 mutations. EXO1-IN-1 holds promise for researching cancers with homologous recombination deficiencies, including BRCA-related tumors.