CD59 is a 19-25 kD glycosylphosphatidylinositol (GPI)-anchored glycoprotein also known as protectin, MACIF, and H19. It is broadly expressed on hematopoietic and non-hematopoietic cells. CD59 inhibits the cytolytic activity of complement by binding to C9, inhibiting incorporation into C5b-8, and preventing the terminal steps in complement polymerization of the membrane attack complex. CD59 has also been reported to play a role in T cell activation.