HGF R (Hepatocyte Growth Factor Receptor), also known as Met, is a receptor tyrosine kinase that plays a central role in epithelial morphogenesis and cancer development. In the absence of ligand, HGF R forms noncovalent complexes with a variety of membrane proteins including CD44v6, CD151, EGF R, Fas, Integrin alpha 6 beta 4, Plexins B1, 2, 3, and MSP R/Ron. Ligation of one complex component triggers activation of the other, followed by cooperative signaling effects. Formation of some of these heteromeric complexes is a requirement for epithelial cell morphogenesis and tumor cell invasion. Paracrine induction of epithelial cell scattering and branching tubulogenesis results from the stimulation of HGF R on undifferentiated epithelium by HGF released from neighboring mesenchymal cells.