MICE carrying mutations at the W (Dominant white spotting) and SI (Steel) loci develop abnormalities in three independent systems: neural crest-derived melanocytes, primordial germ cells and haematopoietic stem cells. Consequently, homozygotes of viable mutant alleles have white coats and are sterile and severely anaemic. Tissue recombination studies predict that the W gene is expressed cell autonomously, whereas the product of the SI locus affects the microenvironment in which the stem cells migrate, proliferate and differentiate^1–6. The W locus encodes the proto-oncogene c-kit, a member of the tyrosine kinase receptor family^7,8.The haematopoietic growth factor SCF (stem cell factor) has been identified as the product of the SI locus and a ligand for c-kit^9–11. Here, we report that SCF is expressed during embryogenesis in cells associated with both the migratory pathways and homing sites of melanoblasts, germ cells and haematopoietic stem cells. Both SCF and c-kit are also expressed in a variety of other tissues, including the brain and spinal cord, suggesting that the receptor–ligand system has additional roles in embryogenesis.