FLT3 and acute lymphoblastic leukemia: While Flt3 is virtually unexpressed in T-ALL derived from our double mutant mice, this does not preclude the possibility that FLT3/ITD is important for expansion of a lymphocytic progenitor, with cooperating somatic mutations and epigenetic changes favoring transformation within a later compartment, where FLT3 activity is dispensable.[29] The identification of FLT3 mutations in a subset of T-ALL patients raises important questions that warrant further investigation.[26, 27]