Mll^PTD/WT:Flt3^ITD/WT mice develop a fatal AML that is transplantable. (A) Survival of WT and single knock-in (Mll^PTD/WT:Flt3^WT/WT or Mll^WT/WT:Flt3^ITD/WT) control mice and mice with the heterozygous alleles of Mll^PTD/WT:Flt3^ITD/WT. (B) Mll^PTD/WT:Flt3^ITD/WT cells from spleen containing AML are serially transplantable and lethal. Serially transplanted cells develop into progressively more aggressive AML with each successive passage.