Fms-related tyrosine kinase 3 ligand(Flt3L) is a single-pass type I membrane protein and consists of 232 amino acids. Flt3L is a hematopoietic four helical bundle cytokine, structurally homologous to stem cell factor and colony stimulating factor. Flt3L synergizes well with a number of other colony stimulating factors and interleukins. Flt3L stimulates the proliferation and differentiation of various blood cell progentiors by activating FLT3.