Flt3/Flk-2 (Fms-like tyrosine kinase 3/fetal liver kinase-2) ligand is a hematopoietic cytokine that plays an important role as a co-stimulatory factor in the proliferation, differentiation, and survival of hematopoietic stem and progenitor cells and the development of the immune system (Lyman et al.; Rosnet et al.). Flt3/Flk-2 ligand, together with stem cell factor and thrombopoietin, is commonly used to promote expansion of primitive hematopoietic cells in culture. In combination with myeloid cytokines such as GM-CSF, G-CSF, or M-CSF, Flt3/Flk-2 ligand enhances the growth and numbers of clonogenic myeloid progenitor cells. In synergy with IL-3, IL-4, IL-7, IL-11, IL-12, IL-15, GM-CSF, and TNF-α, Flt3/Flk-2 ligand regulates the development of various lymphoid progenitor cells, including dendritic cell, B cell, T cell, and NK cell progenitors. In contrast, Flt3/Flk-2 ligand has no significant effect on erythropoiesis or megakaryopoiesis (Drexler and Quentmeier; Wodnar-Filipowicz). Flt3/Flk-2 ligand exists as membrane-bound and soluble isoforms. Both isoforms are biologically active and signal through the class III tyrosine kinase receptor (Flt3/Flk-2, CD135; Rosnet et al.). Flt3/Flk-2 ligand is produced by a variety of cell types, including uncommitted and committed hematopoietic cells and stromal fibroblasts, whereas expression of the Flt3/Flk-2 receptor is restricted to CD34+ hematopoietic stem and progenitor cells. Flt3/Flk-2 receptor is also expressed outside the hematopoietic system in the brain, placenta, and testis (Drexler and Quentmeier; Hannum et al.).