OBJECTIVETo study the molecular basis of O04 allele expression of weak A antigen at the ABO locus.

METHODSSerological assays were performed to characterize the erythrocyte phenotype of a proband. Mutations of the ABO gene were screened with polymerase chain reaction (PCR), reverse-transcription PCR (RT-PCR) and DNA sequencing.

RESULTSThe proband was identified as weak AB phenotype by serological assays. A B101 allele and an O04 allele were identified by gene sequencing, gene cloning and RT-PCR.

CONCLUSIONA B101 allele and an O04 allele were confirmed by the sequencing based typing in a suspicious ABO blood group. The O04 allele is characterized by expression of weak A antigen.