Although the mechanism of action of arsenic trioxide is not completely understood, it causes damage to or degradation of the promyelocytic leukemia protein/retinoic acid receptor-α (PML/RARα) fusion protein; induces apoptosis in acute promyelocytic leukemia cells and many other tumor cell types; promotes cell differentiation and suppresses cell proliferation in many different tumor cell types; and is pro-angiogenic. The gene discussed is PML; the disease is acute promyelocytic leukemia.