CD99, the product of the MIC2 gene, exhibits an erythroid-specific quantitative 
polymorphism coregulated with the polymorphism of the XG blood group gene. As a 
preliminary study of this phenomenon, human XG and CD99 recombinant proteins 
were expressed in murine RAG cells and analyzed by flow cytometry. Both proteins 
were expressed independently and at a similar level in single and double 
transfectants. Immunoprecipitation and Western blot analysis, using the murine 
monoclonal antibodies NBL-1 and 12E7, revealed species of 26 kd (XG) and 32 kd 
(CD99), respectively. A putative 28-kd intracellular precursor of CD99 was also 
detected, as was a 26-kd species after neuraminidase treatment of 
CD99-expressing cells. No evidence of association or complex formation between 
XG and CD99 proteins could be proven, either on transfected RAG cells or on 
human erythrocytes. These results were confirmed using somatic hybrids between 
single transfectants. These findings suggest that the phenotypic relationship 
between XG and CD99 is mostly regulated at the transcriptional level, but they 
do not formally exclude some posttranscriptional effect. Studies on the tissue 
specificity of XG expression showed that surface expression of the XG protein 
could not be restored in somatic hybrids between B-lymphoblastoid cell lines 
from Xg(a+) persons and fibroblasts (RAG) or erythroid (MEL) cells. RT-PCR 
analysis of the transcripts revealed the existence of an XG mRNA in each cell 
line, suggesting that the tissue-specific regulation of cell surface XG 
expression occurs either at a quantitative transcriptional level or is a 
posttranscriptional event. By Northern blot analysis, XG transcripts were 
detected in erythroid tissues and several nonerythroid tissues. (Blood. 
2000;95:1819-1826)