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 508C7>T in exon 4 of DAF in the proband. The proband's daughter was heterozygous for 508C/T. This study describes an Inab phenotype in which the 508C>T nonsense mutation is predicted to change arginine at amino acid residue 136 to a stop codon. This change is in SCR 3 of DAF. This study reports on the molecular basis of a new proband with the Inab phenotype who had no history of intestinal disorders.