Fusion proteins involving the retinoic acid receptor α (RARα) and PML or PLZF nuclear protein are the genetic markers of acute promyelocytic leukemia (APL). APLs with PML-RARα or PLZF-RARα fusion protein differ only in their response to retinoic acid (RA) treatment: the t(15;17) (PML-RARα-positive) APL blasts are sensitive to RA in vitro, and patients enter disease remission after RA treatment, while those with t(11;17) (PLZF-RARα-positive) APLs do not. Recently it has been shown that complete remission can be achieved upon treatment with arsenic trioxide (As₂O₃) in PML-RARα-positive APL, even when the patient has relapsed and the disease is RA resistant. This appears to be due to apoptosis induced by As₂O₃ in the APL blasts by poorly defined mechanisms. Here we report that (i) As₂O₃induces apoptosis only in cells expressing the PML-RARα, not the PLZF-RARα, fusion protein; (ii) PML-RARα is partially modified by covalent linkage with a PIC-1/SUMO-1-like protein prior to As₂O₃ treatment, whereas PLZF-RARα is not; (iii) As₂O₃ treatment induces a change in the modification pattern of PML-RARα toward highly modified forms; (iv) redistribution of PML nuclear bodies (PML-NBs) upon As₂O₃ treatment is accompanied by recruitment of PIC-1/SUMO-1 into PML-NBs, probably due to hypermodification of both PML and PML-RARα; (v) As₂O₃-induced apoptosis is independent of the DNA binding activity located in the RARα portion of the PML-RARα fusion protein; and (vi) the apoptotic process is bcl-2 and caspase 3 independent and is blocked only partially by a global caspase inhibitor. Taken together, these data provide novel insights into the mechanisms involved in As₂O₃-induced apoptosis in APL and predict that treatment of t(11;17) (PLZF-RARα-positive) APLs with As₂O₃ will not be successful.