The PML–RARα fusion protein is the key driver of APL leukemogenesis and the target of ATO.2 The differentiation of human APL cells induced by ATO is related to the degradation of PML–RARα fusion protein.16 In consideration of the similarity between arsenic and selenium, we hypothesized that Se4+-induced differentiation of NB4 cells and primary APL cells might be related to the degradation of PML–RARα fusion protein. The gene discussed is PML; the disease is acute promyelocytic leukemia.