CD59 is a recently discovered complement (C) regulatory protein. Three activities of CD59 have been recognized so far. It can downregulate the activation of the C cascade at membrane attack complex formation stage, it participates in T-cell rosette formation with erythrocytes and appears to be necessary for T-cell activation. CD59 is broadly distributed on cells of various organs and this is compatible with its function of protecting cells and tissues from incidentally activated autologous C. CD59 is encoded by a single gene residing on chromosome 11. The entire sequence of CD59 cDNA is known, from which the amino acid structure of CD59 has been deduced. Mature CD59 is made up of 103 amino acids. It has two potential N-glycosylation sites. Carbohydrate constitutes about 20% of the molecular mass of CD59. The protein part of the molecule is covalently linked to an oligosaccharide which, in turn, is glycosydically linked to phosphatidylinositol (PI). CD59 is anchored to the cells via PI moiety. The fine structure of the PI anchor of CD59 has not yet been established. Decreased expression of CD59 has been shown in two diseases. In paroxysmal nocturnal hemoglobinuria, erythrocytes (type III) lacking CD59 (and other PI proteins) have susceptibility to lysis by membrane attack complex. In psoriasis, expression of CD59 (and CD55) is drastically decreased in psoriatic lesions, presumably due to PI cleavage involving signal transduction mechanism(s) leading to increased proliferation of various cell types in lesional skin.