Genotype-phenotype discrepancy due to tri-allelic ABO genotypes. (A) Serologic test using microcolumn agglutination. In the reaction with monoclonal anti-A antibody, double distinct RBC populations are noted. One major (about 70∼80%) population has the O phenotype, while another minor population has the A phenotype. (B) Direct sequencing analysis revealing the heterozygous indel. Small consecutive phase-shifted peaks are seen before and after the site of the deletion (c.261delG) in the reverse and forward reactions, respectively. (C) STR analysis of this patient showing two major and two minor alleles.