Objective To investigate serological blood typing of the ABO locus which contradict to general law of inheritance in parentage,and the underlying reasons for severe hemolytic disease of newborn(HDN).Methods To research the family whose newborn is AB phenotypes,mother is O phenotypes and father is AB phenotypes.The familiy were genotyped by parentage tests, serological tests,PCR-SSP and direct DNA sequencing at exons 6 and 7 of ABO gene.At the salne time,HDN was detected by micro column gel Coombs (MGCT), and the primary fingerposts of the routine blood tests. Biochemical tests were dynamically observed.Results The results of parentage tests showed that three-generation pedigree have parent-child relationship. The red blood cell(RBC)of this AB phenotypes of this family members strongly agglutinated(4+)with diverse monoelonal anti-A and anti-B antibodies,and their serum did not contain anti-A and anti-B antibodies in blood anti-typing.PCR-SSP can not detect their A and B gene,but DNA sequencing at exons6 and 7 of ABO gene revealed that it had the B(A)803C→G mutation.Conclusions The genetm basis of this parentage are B(A)803G blood gene which harbored both A and B difunctionality of glyeosyhransferases.This was the first report that severe HDN resulting from a large number of A and B antigens in RBC of B(A)phenotype of a newborn,which has clinical significance on ABO locus.