Expression of a bcr-3 isoform of retinoic acid receptor alpha-promyelocytic leukemia (RARalpha-PML) in mice expressing a bcr-1 isoform of PML-RARalpha is associated with increased penetrance of murine acute promyelocytic leukemia (APL) and the frequent acquisition of an interstitial deletion of one copy of mouse chromosome 2 (del(2)). To determine whether the isoform of RARalpha-PML is important for these effects, we created mice that expressed a bcr-1 isoform of RARalpha-PML. Coexpression with the bcr-1 isoform of PML-RARalpha did not increase the penetrance of APL (7 of 45 animals developed APL with PML-RARalpha alone vs 12 of 44 with both transgenes; P=.19). Furthermore, the frequency of del(2) in APL cells from doubly transgenic mice was not different from that of mice expressing PML-RARalpha alone (3 of 6 vs 6 of 12, respectively-P=1.38-compared with 11 of 11 for mice coexpressing PML-RARalpha and bcr-3 RARalpha-PML). The bcr-1 and bcr-3 isoforms of RARalpha-PML, therefore, have different biological activities that may be relevant for the pathogenesis of murine APL.