E2F1 and leukemia: Importantly, these results suggest that the failure of many recipients of Bcr-Abl-transduced E2f1+2+ (particularly at low transduction efficiency) to develop leukemia was not due to our inability to introduce Bcr-Abl into the appropriate progenitor cells, but to the inability of Bcr-Abl+ progenitors to be maintained in the face of replication competent competitors.