The RON receptor tyrosine kinase is a 180kDa heterodimeric protein composed of a 40kDa α chain and a 145kDa β chain with intrinsic tyrosine kinase activity. Activation of RON causes cell dissociation, motility and invasion of extracellular matrices, suggesting that RON might be involved in tumor metastasis. We report here the cloning of a novel splice variant of RON in human colorectal carcinoma cell line HT-29. This RON variant is first produced as a single chain precursor with a molecular mass of 160kDa. Proteolytic cleavage results in a 40kDa α chain and a short form of the β chain with a molecular mass of 125kDa. The altered receptor is synthesized from a transcript differing from the full-length RON mRNA by an in-frame deletion of 109 amino acids in the extracellular domain of the RON β chain. The consequence of the deletion is constitutive activation of the protein with auto-phosphorylation. Expression of the RON variant in colon epithelial CoTr cells results in increased cell migration and invasion of extracellular matrices. These data suggest that generation of the activated splice variant of RON may contribute to the invasive phenotype of human colorectal carcinomas in vivo.