Fms-Related Tyrosine Kinase 3 Ligand (FLT3LG) is a hematopoietic four helical bundle cytokine. Mature human Flt-3 Ligand consists of a extracellular domain (ECD) with a cytokine-like domain and a juxtamembrane tether region, a transmembrane segment, and a cytoplasmic tail. Human and mouse Flt-3 Ligand show cross-species activity. Flt-3 Ligand is expressed as a noncovalently-linked dimer by T cells and bone marrow and thymic fibroblasts.It is structurally homologous to stem cell factor (SCF) and colony stimulating facor 1 (CSF-1). In synergy with other growth factors, Flt3 ligand stimulates the proliferation and differentiation of various blood cell progenitors by activiation of Flt 3 receptor.