The clone F4-29D9 binds with a single chain 70 kDa glycoprotein and identified as CD55 (also known as decay accelerating factor, DAF). CD55 is widely expressed on cells throughout the body including leukocytes, erythrocytes, epithelium, endothelium, and fibroblasts. It is a Glycosyl phosphatidylinositol anchored (GPI-anchored) protein and inhibits autologous complement cascade activation. CD55 also serves as receptor for CD97, echovirus and Coxsackie B virus. Anticd55 is a useful marker for the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH).