FLT3LG (FMS-Related Tyrosine Kinase 3 Ligand) also called FLT3 LIGAND, FL or FLT3L, is a protein which in humans is encoded by the FLT3LG gene. FLT3LG controls the development of DCs and is particularly important for plasmacytoid DCs and CD8-positive classical DCs and their CD103-positive tissue counterparts. Flt3 ligand (FL) is a hematopoietic four helical bundlecytokine. 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. Lyman et al. (1993) found that Flt3 ligand stimulated proliferation of hematopoietic progenitor cells isolated from mouse fetal liver or adult mouse bone marrow. Hannum et al. (1994) concluded that FL enhances the response of stem and primitive progenitor cells to other growth factors to generate all myeloid lineages except erythroid cells. Assays of C-terminally truncated FL proteins confirmed that the N-terminal half conferred FL activity. Depletion of the Pi3k -Mtor negative regulator Pten facilitated Flt3l-driven DC development in culture.