MST1R/Ron, a HGF Receptor/MET-type protein kinase, mediates the biological activities of macrophage-stimulating protein (MSP), a multifunctional cytokine that regulates cell adhesion, motility, growth, and survival. The protein is a membrane-spanning, disulfide-linked heterodimer, which results from cleavage of a glycosylated precursor into 35-kD (alpha) and 150-kD (beta) subunits. Ligand binding results in tyrosine phosphorylation of the beta chain. In knockout studies, MST1R/RON (-/-) mice failed to survive past the periimplantation period. The MST1R/RON gene has been mapped to 3p21, a region of frequent deletion or mutation in small cell lung and renal carcinoma, and has been implicated in the progression of several epithelial cancers.