The Chido/Rodgers blood group system consists of nine antigens, which located on the C4d region of the fourth component of complement (C4). C4d is a tryptic fragment of C4, which become bound to the red cells from the plasma. The encoding gene C4A and C4B are two highly homologous genes, which are remarkable polymorphic distributed in the populations combined with other factors including variant gene copy number and long form/short form gene contribute to the diverse expression of C4A and C4B proteins.