The antigens of the Cromer blood group system reside on decay-accelerating factor (DAF), a protein belonging to the regulators of complement activation family. The blood group system consists of 12 high-prevalence and three low-prevalence antigens. The molecular basis for the antigens is known, and with the exception of IFC, each antigen is the product of a single nucleotide change in the DAF gene and has been localized to one of the four complement control protein (CCP) domains on the DAF protein. The RBCs of people with the Cromer null phenotype, Inab, lack DAF but do not appear to demonstrate increased susceptibility to hemolysis. Antibodies to Cromer antigens are rarely encountered, although there is evidence that the antibodies may cause accelerated destruction of transfused RBCs. There is no risk of HDN associated with Cromer system antibodies because the placenta is a rich source of fetally derived DAF, which is thought to adsorb the antibodies.