To determine whether Flt3 is required for the onset of leukemias induced by MLL fusion genes, BM progenitors from Flt3−/− or Flt3+/+ mice were transduced by MLL-ENL or by an empty MIE (MSCV-IRES-eGFP) retroviral vector and injected into lethally irradiated Flt3+/+ recipient mice. The gene discussed is KMT2A; the disease is leukemia.