Acute promyelocytic leukemia (APL) is an interesting model in cancer research, because it can respond to the differentiation/apoptosis induction therapy using all-trans retinoic acid (ATRA) and arsenic trioxide (AS ₂O ₃). Over the past 5 years, it has been well demonstrated that AS ₂O ₃ induces a high complete remission (CR) rate in both primary and relapsed APL patients (around 85∼90%). The side effects are mild to moderate in relapsed patients, while severe hepatic lesions have been found in some primary cases. After CR obtained in relapsed patients, chemotherapy in combination with AS ₂O ₃ as post-remission therapy has given better survival than those treated with AS ₂O ₃ alone. The effect of AS ₂O ₃ has been shown to be related to the expression of APL-specific PML-RARα oncoprotein, and there is a synergistic effect between AS ₂O ₃ and ATRA in an APL mouse model. Cell biology studies have revealed that AS ₂O ₃ exerts dose-dependent dual effects on APL cells. Apoptosis is evident when cells are treated with 0.5∼2.0 μM of AS ₂O ₃ while partial differentiation is observed using low concentrations (0.1∼0.5 μM) of the drug. The apoptosis-inducing effect is associated with the collapse of mitochondrial transmembrane potentials in a thiol-dependent manner, whereas the mechanisms underlying APL cell differentiation induced by low dose arsenic remain to be explored. Interestingly, AS ₂O ₃ over a wide range of concentration (0.1∼2.0 μM) induces degradation of a key leukemogenic protein, PML-RARα, as well as the wild-type PML, thus setting up a good example of targeting therapy for human cancers.