Many years ago, alert observers noticed among thousands of laboratory mice a few individuals that, unlike their littermates, exhibited areas of white spotting on their fur. No one could have predicted then that an effort to understand the basis for these abnormalities would ultimately contribute to the characterization of a receptor (c-kit) and a corresponding ligand (stem cell factor, SCF) that are critical not only to the migration and development of melanocytes, but also to hematopoiesis, gametogenesis, mast cell development, and, perhaps, development of the central nervous system. Nor could anyone have foretold then that this receptor and ligand would be shown to regulate the development of multiple distinct cellular lineages not only in mice, but also in humans and other primates, or that c-kit and its ligand would be found to influence the secretory function of cells bearing this receptor, as well as their development. Investigation of the effects of SCF on a single cell type, the mast cell, has produced the most complete picture of the spectrum of biological processes that can be regulated by interactions between c-kit and its ligand. This work shows that SCF critically regulates the migration and survival of mast cell precursors, promotes the proliferation of both immature and mature mast cells, enhances mast cell maturation, directly induces secretion of mast cell mediators, and can regulate the extent of mediator release in mast cells activated by IgE-dependent mechanisms. Indeed, SCF may well prove to be one of the most important of the factors influencing mast cell numbers, phenotype, and function in both health and disease. It now seems virtually certain that further studies of c-kit and SCF will produce important new insights into problems as diverse as the regulation of lineage commitment during normal hematopoiesis or the development and function of the central nervous system. And even though an effect on mast cell development was one of the last phenotypic abnormalities to be recognized in mice with mutations affecting the genes encoding c-kit or SCF, mast cells will continue to represent an important model system for analyzing the biology of c-kit and its ligand.