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Xg Blood Group System

Gene locus - XG, CD99


The unique aspect of this blood group system is the location of its locus on the X chromosome at the pseudoautosomal boundary. The gene has been mapped to the distal part of the short arm of the chromosome. It consists of ten exons, the three 5' exons residing in the pseudoautosomal region and the remaining exons within the X-specific end. The latter region is not affected by chromosome inactivation. An apparently duplicated gene, CD99 (MIC2), resides in close proximity within the pseudoautosomal region. Its product, a glycoprotein, CD99 (also known as MIC2,12E7 antigen) bears 48% sequence homology to the XG product. A homologue of XG and MIC genes, XGPY pseudogene, is present on the Y chromosome, at position Yq11.21 The gene is transcribed.

The product of the XG locus, known as Xga glycoprotein, is a 180 amino acid, type II membrane protein, that contains 16 potential sites for O-linked glycosylation. Xga glycoprotein and CD99 are the two antigens that define this system. Only two phenotypes are known, Xg(a+) and Xg(a-); CD99 may be expressed in the latter individuals. It has been proposed that the Xg polymorphism is defined by a difference in the level of expression of the Xg protein at the surface of the erythrocyte rather than by an alteration in the amino acid sequence (Ellisset al.). The XG regulator gene, named XGR located between XG and MIC2 gene, appears to control, at the transcriptional level, the expression of XG and CD99 (Goodfellow et al., Fouchet et al.).

Function of proteins

CD99 is a T cell adhesion molecule. Xg function is not known.

Tissue distribution

CD99 can be detected on essentially all human cells and is highly expressed on all blood cells. Transcripts of Xga have been detected in thymus, bone marrow and fetal liver and several non-erythroid tissues.

Disease association

XG is linked to genes responsible for ichthyosis, occular albinism and retinoschisis; an elevated level of CD99 found in Ewing sarcoma, some neurectodermal tumors, lymphoma and acute lymphoblastic leukemia.

About the alleles

So far, no documentation is available on DNA variation in the XG, MIC2, or the regulator XGY genes. The Xg phenotypic polymorphism may be due to the level of expression of the XG product and/or regulation by XGR (Fouchet et al.). Phenotype: Males | Females - Xg(a+): 65.6 | 88.7; Xg(a-): 34.4 | 11.3

Other database IDs and links

NCBI genes
7499 for XG
4267 for CD99
7501 for XGR
Uniprot ID
P55808 for XG
P14209 for CD99
Genbank proteins
NP_002405 for CD99
NP_780778 for XG
Gene nomenclature database ID
12806 for XG
7082 for CD99
12808 for XGR
Genbank nucleic acids
NM_175569 for XG
NM_002414 for CD99
NCBI homologenes for homologs and orthologs
16288 for CD99
NCBI dbSNP for single nucleotide polymorphisms
7499 for XG
4267 for CD99
7501 for XGR
OMIM ID - at Online Mendelian Inheritance in Man
314700 for XG
313470 for CD99
314705 for XGR


For a recent review see: N.C.Johnson, XG: the forgotten blood group system. Immunohematology 2011 27 68-71

New PubMed entries with the terms Xg and blood from the last 30 days.
NCBI Book Sections with the terms Xg and blood.


Marion Reid and Christine Lomas-Francis, Immunohematology, Laboratory, New York Blood Center, 310 East 67th St., New York, N.Y., 10021

Contributors for specific alleles are listed with the alleles.


Updated 2012-01-09 22:13:44.817
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