SCF, or c-kit ligand or steel factor, is a hematopoietic growth factor that exerts its activity by signaling through CD117, the c-Kit receptor. Soluble and transmembrane SCF is produced by a variety of cells including fibroblasts and endothelial cells and is critical for driving HSC survival, proliferation and differentiation. SCF potentially effects stem cell niche or microenvironment of stem cell location affecting HSC maintenance and along with SDF-1, allows stem cells to relocate into the niche. SCF plays a role in hematopoiesis, spermatogenesis, melanocyte development and other developmental activities. In vitro SCF and c-Kit also increase megakaryocyte development. SCF has clinical value for bone marrow transplantation but also enhances allergic reactions due to mast cell expansion through the CD117 c-kit receptor.