RelevanceTNF RI (CD120a) is a 55 kDa transmembrane glycoprotein that is expressed by virtually all nucleated mammalian cells. Among the numerous cells known to express TNF RI are hepatocytes, monocytes and neutrophils, cardiac muscle cells, endothelial cells, and CD34+ hematopoietic progenitors. Both TNF alpha and TNF beta bind to TNF RI. Soluble TNF alpha binds with a Kda in the range of 20-60 pM, while TNF beta binds with a Kda equal to 650 pM. TNF RI relative to TNF RII seems to be the more physiologically relevant receptor, whereas TNF R2 appears to play a direct role in only a limited number of TNF responses. Soluble TNF RI, which blocks TNF alpha activity, has been identified in both urine and blood (1-3 ng/mL). Serum levels of sTNF receptors increase dramatically in certain pathological situations.