Flt3L, also known as Fms-related tyrosine kinase 3 ligand and SL cytokine, is a single-pass type I membrane protein. It is expressed by stromal cells and T cells. Flt3L signals through tyrosine kinase receptor Flt3/Flk2 to stimulate the proliferation of early hematopoietic progenitor cells. It synergizes with other growth factors, such as GM-CSF, IL-3 and CSF, to promote the differentiation of both myeloid and lymphoid cells. Alternative splicing and proteolytic cleavage of membrane-bound Flt3L generates a soluble extracellular domain (ECD) isoform with full biological activity.