XG, GeneID 7499, stands for Xg glycoprotein or Xg blood group. The XG blood group system includes 2 antigens: Xga and CD99 (encoded by the MIC2 gene). The XG blood group is the first blood group mapped to the human X chromosome. XG is very essential in genome mapping and whole-genome sequencing. In other words, it is an important tool used to examine the human X chromosome as a marker to map sex-linked traits and to help define mechanisms underlying sex chromosome disorders.