Flt3L exists in both a membrane-bound and proteolytically cleaved soluble isoform, both of which are biologically active, and initiates cell proliferation/ differentiation signalling in co-operation with other colony stimulating factors (CSFs) and interleukins, by binding to and activating the tyrosine kinase receptor Flt3 (FLT3R), expressed highly by hematopoietic progenitor cells and stem cells.