The XG blood group system is best known for its contributions to the fields of 
genetics and chromosome mapping. This system comprises two antigens, Xg(a) and 
CD99, that are not antithetical but that demonstrate a unique phenotypic 
relationship. XG is located on the tip of the short arm of the X chromosome with 
exons 1 to 3 present in the pseudoautosomal region of the X (and Y) 
chromosome(s) and exons 4 to 10 located only on the X chromosome. Xg(a) 
demonstrates a clear X-linked pattern of inheritance. MIC2, the gene encoding 
the CD99 antigen, is found in the pseudoautosomal region of both the X and Y 
chromosomes. Anti-Xg(a) is comparatively rare, and only two examples of 
anti-CD99 have ever been identified. Alloanti-Xg(a) is considered clinically 
insignificant; only one example of autoanti-Xg(a) has been reported, but it 
resulted in severe hemolytic anemia. Insufficient data exist to determine the 
clinical significance of anti-CD99. Linkage of XG to several X-borne genes 
encoding inherited disorders has been demonstrated. CD99 is an adhesion 
molecule, and high levels are associated with some types of cancer.