Our results confirmed that the PML/RAR alpha gene on the der(15)t(15;17), not the RAR alpha/PML gene, must be essential to leukemogenesis in APL.     ///    Fluorescence in situ hybridization with cosmid DNA probes flanking the breakpoints of t(15;17) did not show the retinoic acid receptor alpha (RAR alpha)/PML fusion signal usually generated on the der(17)t(15;17). 