Steel factor (SLF) is a recently identified growth factor which is the gene product of the murine Steel locus and a ligand for the c-kit tyrosine kinase receptor, the product of the dominant white spotting locus (W). Defects at these genetic loci result in aberrant melanocyte, germ cell, and hematopoietic development. Both the receptor (c-kit) and the ligand (SLF) have been shown to undergo tissue-specific mRNA splicing to produce distinct isoforms which have unique biological functions. As predicted by the phenotype of these mutations, SLF influences the growth and differentiation of melanocytes, primordial germ cells, and a broad spectrum of cell types in the hematopoietic progenitor and stem cell hierarchy. SLF has also been shown to have effects on hematopoietic lineages not predicted by defects seen in the Steel mouse.