The NOK-1 antibody reacts with human CD178 (Fas ligand) in both membrane bound and soluble forms. Fas ligand is a 40 kDa transmembrane glycoprotein, a member of the TNF-α family, and is expressed by activated T and NK cells, neutrophils, and monocytes. Interactions between CD178 (Fas ligand) and CD95 (Fas) induce a program of apoptosis and play a key role in immune regulation and homeostasis. The extracellular domain of human CD178 can be cleaved from the surface by matrix metalloproteinases (MMPs) resulting in a 26 kDa soluble protein.