CD59, an 18-20 kDa GPI-anchored glycoprotein, is broadly distributed, present in most tissues, and on all circulating cells. CD59 has been also known as 20 kDa homologous restriction factor (HRF20), membrane attack complex inhibitory factor (MACIF), membrane inhibitor of reactive lysis (MIRL) and protectin. CD59 inhibits the cytolytic activity of complement by binding to C8 and C9 in the assembling of the cytolytic membrane attack complex (MAC), thereby interfering with membrane insertion and pore formation. CD59 and other membrane complement regulatory proteins (CD46 and CD55) are frequently overexpressed on cancer cells, possibly as a mechanism for cancer cells to overcome lysis by complement. CD59 also interacts with T cell-restricted glycoprotein CD2 and plays a role in T cell activation.Synonyms: 20 kDa homologous restriction factor, HRF-20, HRF20, MAC-IP, MAC-inhibitory protein, MACIF, MEM43 antigen, MIC11, MIN1, MIN2, MIN3, MIRL, MSK21, Membrane attack complex inhibition factor, Membrane inhibitor of reactive lysis, Protectin