Known hybrid alleles at the ABO locus. The common alleles are also shown for comparison. Only changes from the consensus (A1-1) sequence are shown. Mutations causing amino acid changes are shown in bold face. The yellow areas indicate a reading frame shift. The blue areas indicate untranslated regions. The introns are represented by the thick, dark vertical bars. The blue rectangles indicate the region of the allele where a crossing over event occurred. ?, indicates that the nucleotide at this position was not described.