FLT3LG is a hematopoietic cytokine whose activities are mediated by binding to the transmembrane glycoprotein Flt3. Flt3 was first discovered as a member of the class III subfamily of receptor tyrosine kinases (RTK) whose expression among hematopoietic cells was found to be restricted to highly enriched stem/progenitor cell populations. Additionally, class III RTKs include the receptors from SCF, CSF3 and PDGF. Not surprisingly, FLT3LG is also structurally related to CSF3 and SCF. All three cytokines have been shown to exist both as type I transmembrane proteins and as soluble proteins. The predominant human FLT3LG isoform is a transmembrane protein that can undergo proteolytic cleavage to generate a soluble form of the protein. An alternatively-spliced FLT3LG mRNA, encoding a soluble form of the human FLT3LG, has also been identified. FLT3LG is widely expressed in various human and mouse tissues. At the amino acid sequence level, human and mouse FLT3LG are approximately 72% identical and the two proteins exhibit cross-species activity. FLT3LG has been shown to synergize with a wide variety of hematopoietic cytokines to stimulate the growth and differentiation of early hematopoietic progenitors.