E2F1 and leukemia: Finally, while it remains formally possible that DKO stem/progenitor cells home poorly to BM niches and that Bcr-Abl restores homing to DKO progenitors, the fact that a low percentage of cotransplanted E2f1+2+ BM can reverse the selective expansion of Bcr-Abl+ DKO progenitors and leukemogenesis (Figures 4 and S1) strongly argues against a role for differential homing in genotype-dependent leukemia rates.