Flt-3 ligand (FL) is a member of a family of cytokines that stimulate the proliferation of hematopoietic cells. Other cytokines in this family include stem cell factor (SCF, also known as c-kit ligand. steel factor or mast cell growth factor) and colony-stimulating factor 1 (CSF-1, also known as macrophage colony-stimulating factor (M-CSF)). All three of these cytokines belong to the 'helical cytokine' family, whose members have o-helical bundles in their structure. FL, SCF, and CSF-1 bind to type III tyrosine kinase receptors (FL with Flt-3 receptor; SCF with c-kit; and CSF-1 with c-fms) from the platelet-derived growth factor receptor (PDGFR) subfamily of tyrosine kinase receptors (Lyman. 1998; Lyman and Jacobsen. 1998; Shurin et al., 1998; Antonysamy and Thomson. 1999).