The Xg^a and CD99 blood group phenotypes. (A) The Xg^a and CD99 blood group phenotypes of the 78 Taiwanese enrolled in this study. A total of 41 of the 47 females and 19 of the 31 males were found to have the Xg(a+) phenotype, and the remaining 6 females and 12 males were found to have the Xg(a−) phenotype. (B) Determination of the CD99H and CD99L blood group phenotypes using flow cytometry. The left and right panels were obtained from the flow cytometry analyses of RBCs derived from 1 Xg(a+) female and 1 Xg(a−) female, respectively. The open and shaded areas represent the results obtained from RBCs incubated with monoclonal antibody 12E7 and FITC-conjugated second antibody and with FITC-conjugated second antibody only, respectively. (C) Quantitative comparison of CD99 expression levels across the individual groups of the various Xg^a/CD99 blood groups in females and males. The geometric mean of the fluorescence intensity (MFI) of CD99 for each sample was obtained from the flow cytometry analysis by subtracting the MFI of the RBCs incubated with the second antibody only from the MFI of the RBCs incubated with the 12E7 mAb, followed by the second antibody. The means of CD99 MFI in each of the individual groups according to their Xg^a/CD99 phenotypes in females and males are presented; the error bars indicate the standard error of the mean of each group. Statistical analyses among the different phenotypic groups of females and males were performed using the Mann-Whitney U test (****P < .0001) and the Kruskal-Wallis test (***P < .001), respectively. The results showed that the CD99 expression levels of the Xg(a+)/CD99H groups were significantly higher than those of the Xg(a−)/CD99L groups, and the CD99 expression level of the Xg(a−)/CD99H male group was found to be intermediate between the levels of the Xg(a+)/CD99H males and Xg(a−)/CD99L males.