Mice of genotype W/Wv and Sl/Sld have been considered as a model of instinct hemopoietic disorders. W/Wv mice have a defect in hemopoietic stem cells and Sl/Sld mice have a defect in the microenvironment. The W locus in murine chromosome 5 encodes the c-kit proto-oncogene and the Sl locus in chromosome 10 encodes the ligand for c-kit, which has been named stem cell factor (SCF), mast cell growth factor (MGF), kit ligand (KL) and steel factor (SL). The cDNA sequence of SCF suggest that it is synthesized as an integral transmembrane protein and that it has common tertiary structure with M-CSF. SCF enhances the proliferation of hemopoietic stem cells and progenitor cells as well as mast cell in combination with other growth factors. Furthermore, it plays an important role in the proliferation and migration of embryonic stem cell, primordial germ cell and melanocyte.