Fms-related tyrosine kinase 3 ligand (Flt3-ligand) is a growth factor that regulates early hematopoiesis. Flt3-ligand belongs to a small family of α-helical cytokines. In synergy with other growth factors like G-CSF, GM-CSF, SCF, and IL-3 Flt3-ligand stimulates the proliferation and differentiation of primitive hematopoietic stem cells. Early B cell lineage differentiation as well as expansion of monocytes and immature dendritic cells is stimulated. Flt3-ligand is expressed by T lymphocytes and bone marrow stromal fibroblasts as a membrane-bound and a soluble isoform. Both isoforms signal through the tyrosine kinase receptor Flt3/Flk-2, which is restricted to cells of hematopoietic origin. Mouse Flt3-Ligand is also active on human cells.