CD59, also known as protectin or membrane attack complex inhibitory factor (MACIF), is a glycosylphosphatidylinositol (GPI)-anchored membrane protein that plays a crucial role in regulating the complement system. It is widely expressed in most human tissues and acts as a major inhibitor of the membrane attack complex (MAC) formation. The mature CD59 protein consists of 77 amino acids and is anchored to the cell membrane via a GPI anchor. By binding to complement proteins C8 and C9, CD59 prevents the assembly of the MAC, thereby protecting host cells from complement-mediated lysis. Deficiencies in CD59 expression can lead to serious disorders such as paroxysmal nocturnal hemoglobinuria (PNH), characterized by the destruction of red blood cells. Additionally, CD59 is involved in T cell signal transduction and has been implicated in various diseases, including cancer and autoimmune disorders.