Fms-related tyrosine kinase-3 ligand (Flt-3 Ligand) is a protein which is encoded by the FLT3LG gene in human. As an important regulator of hematopoiesis, Flt-3 ligand has a tyrosine-protein kinase activity that stimulate the proliferation of hematopoietic progenitor cells of both lymphoid and myeloid origin. Flt-3 ligand can synergistically increase the proliferation of immature progenitors with other cytokines such as GM-CSF, G-CSF, IL-3, EPO, IL-11, IL-12, IL-6 or TPO. Flt-3 ligand is expressed by a variety of hematopoietic progenitor cells including immature hematopoietic and