Sixty cis-AB01 donors were identified. cis-AB01/O01 or O02 were the most common genotypes (36/60) detected only in A(2)B(3) donors, and cis-AB01/B101 (nine of 60) was the least common genotype identified only in A(2)B donors. Surprisingly cis-AB01/A102 (15/60) was identified in a variety of phenotypes (A(1)B(3), A(1)B(x) or el, A(int)B(3)).

A serological and genetic study of Korean blood donors with phenotypic group A subtypes was performed. There were 176 donors with phenotypic A subtypes identified. Exons 6 and 7 from 57 representative donors were sequenced. The A(var) allele (784 G > A) was cloned and sequenced, and a family study demonstrating its inheritance and unusual serological characteristics was performed. The A102 allele was the most frequently identified allele in phenotypically A2 (58%, 11/19) and A2B (68%, 17/25) donors. Anti-A1 was rarely present amongst A2 and A2B donors. The family study revealed that the A(var) allele was expressed as phenotype A(weak)B in A(var)/B heterozygote members, but as phenotype O in A(var)/O heterozygotes. The most frequent allele in Korean donors with the A2 phenotype differs from its Caucasian counterpart, as does the frequency of anti-A1. The A(var) allele demonstrates allelic enhancement in A(var)/B heterozygotes.

B(var) allele is subject to differential expression, depending on the co-inherited ABO allele.

Cis-AB enzymes feature variable but reduced GTA and GTB activities with relatively weaker GTB activity, consistent with the weak agglutination present on forward typing with anti-B.

Numerous examples of A or B subgroup alleles have been described in many geographical and ethnic groups. To analyse the phenotypic and genotypic basis of some ABO subgroups in Korea, a large-scale study was performed on 169 605 samples from Korean blood donors. The overall incidence of ABO subgroups was 0Á13% (227/169,605) in this donor population. Based on genotyping and/or serologic typing, the subgroups identified were as follows: A 2 B (n = 120), A 2 B 3 (n = 36), A 1 B 3 (n = 22), A 2 (n = 13), B 3 (n = 13), A int B 3 (n = 6) and others (n = 17). Amongst these individuals with ABO subgroups, 26Á4% (60/227) demonstrated a cis-AB01 allele. It has been reported that the Korean population has several unique ABO subgroups (Aw10, Aw14 and B306) showing allelic competition or allelic enhancement. Several congenital blood chimaeras have been also found when studying individuals with ABO subgroups. Like ABO subgroups showing the allelic enhancement and allelic competition, chimaerism is also an important cause of ABO phenotype-genotype discrepancies when implementing DNA-based blood grouping techniques. To understand the pattern of A and B antigen expression amongst donors with subtype alleles, flow cytometric analysis of ABO antigen expression on HeLa cells transfected with plasmids containing various mutants was used, in addition to ABO genotyping. ABO subgroup frequencies in KoreaThe human ABO blood group is determined by the ABO gene located on chromosome 9. This gene contains seven exons, of which the two largest exons (6 and 7) comprise 77% of the coding sequence of the gene and encode 91% of the catalytic domain. The gene encoding a glycosyltransferase (GT) can transfer either N-acetylgalactosamine (GalNAc) to create an A antigen (glycosyltransferase A, GTA) or galactose (Gal) to create a B antigen (glycosyltransferase B, GTB) to the acceptor H substance [1-4]. Of the common ABO alleles, A alleles (A101 and A201 are common alleles in Caucasians, but not in Asians; A102 is a common allele in Asians), B allele (B101) and O alleles (O01, O02 and O03; O03 is rare in Asians) have been reported [5][6][7][8][9][10][11]. In addition to the common ABO blood types, weak subgroup phenotypes (A 2 , A 3 , A x , A el , B 3 , B x , B(A) and cis-AB) have been identified [7]. It is well known that the phenotypic frequencies and genetic basis of ABO subgroups differ between ethnic regions. To analyse the ABO subgroup frequencies in Korea, a large-scale study was performed on 169 605 samples from Korean blood donors. The overall incidence of ABO subgroups was 0Á13% (227/169,605) in this donor population. Based on genotyping and/or serologic typing, the subgroups identified were as follows: A 2 B (n = 120), A 2 B 3 (n = 36), A 1 B 3 (n = 22), A 2 (n = 13), B 3 (n = 13), A int B 3 (n = 6), A w B (n = 5), A 1 B x or el (n = 5), A int (n = 2), A m or el (n = 2), A el (n = 2) and B m or x (n = 1) [12]. Cis-AB blood group in KoreaAmongst 0Á13% (227/169 605) ABO subgroups in the large-scale study of Korea, 26Á4% (60/227) had a cis-AB ...