The Cromer blood group system consists of ten high-prevalence and three low-prevalence antigens carried on decay-accelerating factor (DAF). DAF is found in the cell membranes of RBCs, granulocytes, platelets, and lymphocytes and is widely represented in other body tissues. Sequence analyses of DNA were performed on a blood sample from a 91-year-old Japanese woman whose serum contained an alloantibody to a high-prevalence antigen in the Cromer blood group system (anti-IFC). A blood sample from her daughter was also studied. Sequence analysis revealed a substitution of 508C>T in exon 4 of