Background：

This gene is a member of the receptors of complement activation (RCA) family and is located in the ’cluster RCA’ region of chromosome 1. The gene encodes a monomeric single-pass type I membrane glycoprotein found on erythrocytes， leukocytes， glomerular podocytes， and splenic follicular dendritic cells. The Knops blood group system is a system of antigens located on this protein. The protein mediates cellular binding to particles and immune complexes that have activated complement. Decreases in expression of this protein and/or mutations in its gene have been associated with gallbladder carcinomas， mesangiocapillary glomerulonephritis， systemic lupus erythematosus and sarcoidosis. Mutations in this gene have also been associated with a reduction in Plasmodium falciparum rosetting， conferring protection against severe malaria. Alternate allele-specific splice variants， encoding different isoforms， have been characterized. Additional allele specific isoforms， including a secreted form， ha