The regular RIN1 gene is a molecule located on chromosome 1lq13.2, and contains a coding region of 2352 bp with a 3' domain that binds to H-Ras protein, suggesting that it is an important molecule in the intracellular signaling pathway. In this study, we confirmed the existence of a novel form of the RIN1 gene with a different splicing pattern, successfully cloned it, and examined its expression in gastric and colon cancer cell lines. A 612-bp band (the RIN1 variant mRNA) was identified in the RT-PCR product from the colon cancer cell line Colo320D. (A 2352-bp band representing the regular RIN1 gene in HT29 cell line.) The 612-bp band was sequenced and compared with that of the regular RIN1 gene. As a result, the 612-bp product was found to contain a tyrosine phosphorylation site on the 5' side and Ras and 14-3-3 binding domains on the 3' side, indicating that it is a product with a different splicing pattern. The expression of the RIN1 variant mRNA was observed in two of six gastric cancer cell lines and four of five colon cancer cell lines. We identified a novel RIN1 gene with a splicing pattern different from that of the regular RIN1 gene. Comparison of both genes revealed that the novel RIN1 products had a structure conserving the Ras and 14-3-3 binding domains, but lacking two tyrosine phosphorylation sites. Novel RIN1 variant protein was expressed primarily in the cytoplasm and no expression in the cell membrane, and RIN1 variant protein was bound to 14-3-3 protein. In addition, the novel RIN1 mRNA was found to be expressed in gastric and colon cancer cell lines, suggesting that it is an important gene for the function of cancer cells.