Acute promyelocytic leukemia (APL) accounts for about 10% of all acute myeloid leukemias and is characterized by the chromosomal translocation t(15;17), which fuses the retinoic acid receptor (RAR) alpha gene to the promyelocytic leukemia (PML) gene. The PML-RAR alpha fusion gene plays an important role in leukemogenesis through antagonizing retinoic acid signalling and the regulatory pathways mediated by PML. APL is the first example of a human cancer that can be effectively treated with the differentiation inducer all-trans retinoic acid (ATRA). The therapeutic effect of ATRA in APL has been associated with the direct modulation of PML-RAR alpha, the restoration of the differentiation pathways regulated by wild-type RAR/retinoid X receptor heterodimer and PML. More recently, a second drug, arsenic trioxide (As2O3), has been discovered in China that also has a strong therapeutic effect against APL. As2O3 can induce clinical remission in de novo or relapsed APL patients and has no cross-resistance with ATRA. It has dual effects on APL cells: preferential apoptosis at high concentration (0.5-2 microM) and partial differentiation at low concentration (0.1-0.5 microM). Modulation and degradation of PML-RAR alpha proteins can be induced by As2O3 and probably contribute to these two effects. These studies lead to a model in which PML-RAR alpha could be the target of both ATRA differentiation therapy and As2O3 apoptosis therapy.