Flt3-Ligand belongs to a small group of growth factors that regulate proliferation of early hematopoietic cells. Multiple isoforms of Flt3-Ligand have been identified. The predominant form is the transmembrane which is biologically active on the cell surface. The proteolytically cleaved transmembrane isoform generates a soluble form, which is also biologically active. Flt3-Ligand binds to cells expressing the tyrosine kinase receptor Flt3. Flt3-Ligand alone cannot stimulate proliferation, but synergizes well with other CSFs and interleukins to induce growth and differentiation. Recombinant murine flt3-Ligand is a 16.4 kDa protein containing 144 amino acid residues.