Schematic representation of single missense amino acid substitutions in A and B glycosyltransferases associated with decreased expression of A and B antigens, respectively.

The 2 horizontal bars represent 20 of the variant A (upper) and B (lower) glycosyltransferases in Tables 5 and 6 with the addition of 4 A₂ transferases.49 CisAB andB(A) alleles were not included. Vertical lines separate regions coded for by different exons (numbered 1-7) in theABO gene. The putative transmembrane domain (amino acid 15-39 of the glycosyltransferase) is shown in gray.5 The arrows indicate the stem region susceptible to proteolytic cleavage for generation of soluble enzyme. All amino acid changes published to date are shown as geometric symbols above the bars (except for P156L, which does not affect glycosyltransferase activity). Filled symbols represent substitutions involving a charged amino acid and open symbols involve noncharged residues. Different ABO phenotypes are represented by the symbols as follows: ▪ and ■, A₂; ▴, A₃/B₃; ● and ○, A_x, A_w/B_w; ♦ and ⋄, A_el/B_el. The A^x alleles other than A^x-1 (hybridsA^x-2 to A^x-5 andA^x-6 with a nonsense mutation at nt 996, Table5) have been excluded from this figure, as has the frame-shiftingA^el-1 allele. Three alleles each having 2 mutations resulting in amino acid changes were included: a, this diamond represents A^el-2 that also has the neutral P156L change; b, this triangle represents a V277M substitution in A3 -2 that also has the common A₂-related mutation (1060C−); c, one of the 3 squares represents the common A2 -1 allele (a nonsense, not missense, mutation).