Macrophage stimulating protein receptor, encoded by the human RON and the mouse Stk genes, is one of a family of receptor tyrosine kinases that also includes human Met. This family of receptors is synthesized as a single-chain precursor that is cleaved into a mature disulfide-linked heterodimer composed of an extracellular a chain and a membrane spanning b chain with intrinsic tyrosine kinase activity. Expression of MSP R is restricted to specific areas of the central and peripheral nervous systems, epithelial cells along the digestive tract, skin and lung, and in subpopulations of the mononuclear phagocyte lineage. Both the MSP heterodimer and the free MSP b chain have been shown to bind MSP R. However, only the MSP heterodimer can induce receptor dimerization and phosphorylation and cause biological activity.