The receptor tyrosine kinases (RTKs) c-kit and platelet-derived growth factor receptor alpha chain (PDG-FRa) are encoded at the white spotting (W) and patch (Ph) loci on mouse chromosome 5. While W mutations affect melanogenesis, gametogenesis, and hematopoiesis, the Ph mutation affects melanogenesis and causes early lethality in homozygotes. W-sash (Wsh) is an expression mutation and blocks c-kit expression in certain cell types and enhances c-kit expression in others, including at sites important for early melanogenesis. We have determined the effect of Ph on c-kit expression during embryogenesis in Ph heterozygotes. Immunohistochemical analysis revealed enhanced c-kit expression in several cell types, including sites important for early melanogenesis. We propose that in both Wsh and Ph mutant mice c-kit misexpression affects early melanogenesis and is responsible for the pigment deficiency. Moreover, we have defined the organization of the RTKs in the W/Ph region on chromosome 5 and characterized the Wsh mutation by using pulsed-field gel electrophoresis. Whereas the order of the RTK genes was determined as Pdgfra-c-kit-flk1, analysis of the Wsh mutation revealed that the c-kit and Pdgfra genes are unlinked in Wsh, presumably because of an inversion of a small segment of chromosome 5. The Ph mutation consists of a deletion including Pdgfra and the 3' deletion endpoint of Ph lies between Pdgfra and c-kit. Therefore, positive 5' upstream elements controlling c-kit expression in mast cells and some other cell types are affected by the Wsh mutation and negative elements are affected by both the Wsh and the Ph mutation.