The R3-5G4 monoclonal antibody specifically binds to Runt-related transcription factor 3 (RUNX3) which is also known as Acute myeloid leukemia 2 protein (AML2), Core-binding factor subunit alpha-3 (CBFA3), and Polyomavirus enhancer-binding protein 2 alpha C subunit (PEBP2aC). RUNX3 is a member of the RUNX transcription factor family which includes RUNX1-3. RUNX3 is a key regulator of gene expression related to the development and differentiation of cells within the nervous and immune systems. In the immune system, RUNX3 is particularly involved in the commitment of CD8+ T cells in the thymus. RUNX3 is highly expressed and essential for the cytotoxic functions of NK cells, peripheral CD8+T cells and CD4+CD8αα intraepithelial lymphocytes in the gut. RUNX3 also plays a role in the differentiation and effector functions of Th1 cells. RUNX3 is activated downstream of the TGF-β signaling pathway and can play a role in tumor suppression. Aberrant expression of RUNX3 has been associated with tumorigenesis including the development of gastric and other cancers.