TWEAK is a TNF superfamily transmembrane protein which is expressed in many different cells and tissues, such as the heart, spleen and small intestine. Two different receptors have been identified that bind TWEAK; the Death Receptor 3 (DR3) and the TWEAK Receptor (TWEAKR). While DR3 is expressed mainly in tissues that have high concentration of lymphocytes, the mitogen-inducible TWEAKR is has been shown to be expressed in fibroblasts, hepetocellular carcinomas and endothelial cells. Recombinant human TWEAK comprises the TNF homologous (secreted region) of TWEAK.