The B allele with a 5·8 kb deletion in intron 1 of the ABO gene is the major allele in Japanese individuals with Bm and A1 Bm phenotypes

Bm and A1 Bm phenotypes are the most frequent ABO variants in the Japanese population. The B antigen on Bm red blood cells is only detectable by adsorption and elution tests, and plasma B-transferase activity is usually detected at half or less levels compared with that of common B. Recently, a B allele lacking an erythroid cell-specific transcription enhancer in intron 1 of the ABO gene was identified from individuals with Bm and A1 Bm phenotypes, which could explain the unique serologic properties of Bm . In the Japanese Red Cross Society, eight Blood Centers tested blood samples from donors throughout Japan and collected blood samples from 888 Bm and 415 A1 Bm individuals. DNA analysis revealed that 1300 of 1303 (99·77%) individuals had the B allele with a 5·8 kb deletion (c.28 + 5110_10889del), which included the enhancer element.