Tumor Necrosis Factor Receptor-I (TNF RI, also known as p55/p60) is a high affinity receptor for TNF-alpha and -beta, potent mediators of multiple aspects of inflammatory immune responses. TNF RI, along with TNF RII, is a prototypic member of the TNF receptor superfamily and has been designated TNFRSF1A. All cell types are thought to express TNF RI, and most of the biological functions of TNF are mediated via TNF RI. Soluble forms of both TNF RI, generated as a result of proteolytic cleavage of the extracellular domain, has been detected in vivo in various biological fluids. The soluble TNF RI receptor can bind TNF with high affinity and functions as a TNF antagonist.