FLT3 Ligand is a member of a small family of growth factors that stimulate the proliferation of hematopoietic cell. FLT3 ligand is known to induce profound expansion of dendritic cells (DCs) in the peripheral lymphoid organs, and FLT3 ligand-induced DCs have an immature phenotype. Its receptor, FLT3, is expressed on myeloid, lymphoid and dendritic cell progenitors and is considered an important growth and differentiation factor for several hematopoietic lineages. Activating mutations of FLT3 are frequently found in acute myeloid leukemia (AML) patients and associated with a poor clinical prognosis. Serum FLT3 Ligand is a biomarker of progenitor cell mass and prognosis in acute myeloid leukemia.