Many spontaneous, chemical-induced, and radiation-induced dominant white spotting (W) and steel (Sl) mutations have been identified in the mouse. W and Sl mutations have similar phenotypic effects including deficiencies in pigment cells, germ cells, and blood cells, Numerous studies have suggested that W acts within the affected cell while Sl instead exerts its effects in the extracellular environment. Recent findings demonstrating that W encodes the c-kit proto-oncogene, a tyrosine kinase membrane receptor, have suggested that Sl encodes a ligand for c-kit. In the accompanying article we report the identification and purification of mast cell growth factor (MGF), a c-kit ligand. Here we describe the cloning of sequences encoding MGF. Furthermore, we show that Mgf maps near Sl in the distal region of mouse chromosome 10 and is deleted in a number of Sl alleles. These findings strongly support the notion that Sl encodes the mast cell growth factor.