We sought to investigate the apoptosis-inducing activities of quercetin, Siamois 1, and Siamois 2 against invasive estrogen-receptor negative MDA-MB 435 cells xenografted in athymic nude mice. This study clearly demonstrated that these compounds exhibited apoptosis-inducing activities in cell culture system. Quercetin (20 μg/mL), Siamois 1 (100 μg/mL), and Siamois 2 (200 μg/mL) can induce apoptotic cell death by 40± 5%, 44 ± 14 %, and 31 ± 13 %, respectively. Two-fold of IC50 of these compounds were clearly found to induce apoptosis in breast tumor tissue which can be determined by 99mTc-annexin V scintigraphy and histological staining. This is the first report that the apoptosis-inducing effects of quercetin, Siamois 1, and Siamois 2 on the MDA-MB 435 cell in vitro were... effectively extrapolated to the in vivo situation. These compounds might be considered as a simple dietary supplement and with further clinical investigation for their use as a nutrition-based intervention in breast cancer treatment.