RUNX3 belongs to the runt-domain family of transcription factors that regulates gene expression in major developmental pathways, such as T-cell differentiation and neurogenesis. RUNX3 is epigenetically silenced in many types of cancers. In gastric cancer, RUNX3 has been frequently reported to be inactivated by gene silencing or protein mislocalization. The oncoprotein SRC phosphorylates RUNX3 at multiple tyrosine residues, resulting in cytoplasmic mislocalization of RUNX3. RUNX3 is regulated by the TGF-B/Smad pathway, and induces apoptosis or cell cycle arrest of gastric cancer cells through upregulating expression of p21 and Bim. The loss of RUNX3 function in T-cells cause suppression of Treg function and formation of colitis tumor. These findings suggest an important role of RUNX3 in tumor suppression.