The RON receptor tyrosine kinase (RTK; also known as MST1R) was first identified in 1993 in a cDNA library from human epithelial cells. RON belongs to the c-MET family of proto-oncogenes. This RTK family has only two members, RON and Met, with only 34% similarity overall; however, the tyrosine kinase regions of the receptors were quite similar at 80% similarity.