CD55, also known as DAF or decay acceleration factor, is a 70-75 kDa member of the RCA protein family that is characterized by four to 30 SCRS (short consistent repeats) in its plasma exposure area. It prevents the assembly of the C3bBb complex (the C3-convertase of the alternative pathway) or accelerates the disassembly of preformed convertase, thus blocking the formation of the membrane attack complex. DAF seems to exert different functions beyond its immunological role such as promotion of tumorigenesis, decrease of complement mediated tumor cell lysis, autocrine loops for cell rescue and evasion of apoptosis, neoangiogenesis, invasiveness, cell motility.