Schematic representation of the genomic organization of the ABO gene. The exons (▪) and regulatory regions (□) are drawn to scale, as are the intervening introns, although the scale of the latter is 10 times smaller. The calculated numbers of nucleotides (nts) in the exons and introns are shown. The upstream regulatory region, which includes a CBF/NF-Y binding motif, is located around nt –3800 and, depending on the ABO haplotype, 215 or 344 base pairs (bp) in size; the regulatory region in the 5′ untranslated region (UTR) is located from nt –118 to –1.