The XG gene encodes the Xg glycoprotein, a blood group antigen located on the X chromosome X expression controlled through a pseudoautosomal region 1. XG comprises two related antigens: Xg(a) and CD99, which demonstrate a unique phenotypic relationship rather than antithetical properties 1. The gene spans the pseudoautosomal boundary, with exons 1-3 in the pseudoautosomal region shared with the Y chromosome X exons 4-10 X-specific, resulting in X-linked inheritance patterns 1. CD99, the product of the MIC2 gene co-regulated with XG, functions as an adhesion molecule and exhibits erythroid-specific quantitative polymorphism with XG(a) expression 2. At the cellular level, CD99 is a plasma membrane protein involved in homotypic cell-cell adhesion and immune cell extravasation, including positive regulation of neutrophil and T cell extravasation 1. Clinically, anti-Xg(a) antibodies are comparatively rare and considered clinically insignificant in transfusion contexts, though one autoanti-Xg(a) case resulted in severe hemolytic anemia 1. The XG system has historically contributed significantly to genetics and X chromosome X studies. CD99 shows elevated expression in certain cancer types, suggesting potential roles in malignant transformation 1.