CD59 is a cell surface glycoprotein that regulates complement-mediated cell lysis, and it is involved in lymphocyte signal transduction. Complement activation ultimately leads to formation of the cytotoxic, poreforming membrane attack complex (MAC), the main effector of complement-mediated tissue damage. Insertion of the MAC into cell membranes induces the release of cytokines and growth factors that promote inflammation, thrombosis, and cell proliferation. CD59 is a complement regulatory protein ubiquitously expressed on mammalian cell surfaces and it specifically inhibits MAC formation and thereby protects “self” cells from complement-mediated damage. The protein also plays a role in signal transduction pathways in the activation of T cells. Mutations in the CD59 gene cause CD59 deficiency, a disease resulting in hemolytic anemia and thrombosis, and which causes cerebral infarction. CD59 was demonstrated to be present in plasma/serum of healthy individuals in levels ranging from 22 to 119 ng/ml.