FLT3 is a receptor tyrosine kinase of 130-155 kDa expresssed on normal bone marrow stem and early progenitor cells and on leukemic blasts from patients with acute leukemias. The FLT3 ligand, FL, is a new cytokine which acts on hematopoietic progenitors in synergy with other cytokines. FLT3 transduces FL-mediated signal through interaction with a number of cytoplasmic substrates.