Camptothecins constitute a novel class of chemotherapeutics that selectively target DNA topoisomerase I (Top1) by reversibly stabilizing a covalent enzyme-DNA intermediate. This cytotoxic mechanism contrasts with that of platinum drugs, such as cisplatin, which induce inter-and intrastrand DNA adducts. In vitro combination studies using platinum drugs combined with Top1 poisons, such as topotecan, showed a schedule-dependent synergistic activity, with promising results in the clinic. However, whereas the molecular mechanism of these single agents may be relatively well understood, the mode of action of these chemotherapeutic agents in combination necessitates a more complete understanding. Indeed, we recently reported that a functional homologous recombination pathway is required for cispla- TPT and CPT-11 have been approved for first and second line treatment of advanced colorectal cancer and second line treatment for ovarian cancer (7-9). As with most chemotherapeutics, these CPT analogs are administered in combination with other agents (10 -13). Although the molecular mechanisms of single agents may be well understood, the combination of two or more agents often induces surprising interactions, which necessitates a more complete understanding of the cytotoxic lesions induced by such combinations for optimal efficacy. For example, in vitro combination experiments using platinum agents, such as cisplatin (cDDP), carboplatin, or oxaliplatin, with a Top1 poison, TPT or CPT-11, demonstrated a sequencedependent synergy in various human tumor cell lines (14 -16). The most cytotoxic sequence was a platinum drug followed by a Top1 poison. Moreover, chemotherapeutic regimens that combine TPT/CPT-11 with platinum drug are showing some promise in the clinic (see Refs. 17-19 and references therein). However, the lack of insight into the mechanism of synergy hampers the optimal design of clinical studies based on the combination of platinum drugs and Top1 poisons.We recently reported genetic evidence that homologous recombination (HR) is responsible for the synergistic cytotoxicity of certain drug/Top1 poison combinations (20). Indeed, a functional HR pathway was required for the synergistic activity of platinum or X-irradiation in combination with TPT yet acted to suppress the synergistic combination of 1--D-arabinofuranosyl cytidine