Involved in complement regulation. The dimerized forms have avidity for tissue-bound complement fragments and efficiently compete with the physiological complement inhibitor CFH. Head-to-tail homodimer and heterodimer with CFHR1 or CFHR2. Binds C3b in vitro. Expressed by the liver and secreted in plasma. Defects in CFHR5 have been found in patients with atypical hemolytic uremic syndrome and may contribute to the disease. Atypical hemolytic uremic syndrome is a complex genetic disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, renal failure and absence of episodes of enterocolitis and diarrhea. In contrast to typical hemolytic uremic syndrome, atypical forms have a poorer prognosis, with higher death rates and frequent progression to end-stage renal disease. Susceptibility to the development of atypical hemolytic uremic syndrome can be conferred by mutations in various components of or regulatory factors in the complement cascade system. Other genes may play a role in modifying the phenotype. The disease is caused by variants affecting the gene represented in this entry.