Xg Proteins and Enzymes

Xg is a 26 kDa, type I integral membrane glycoprotein which belongs to the CD99 family. It is found in erythrocytes and is responsible for the XG blood group system. Its presence or absence determines Xg(a+) and Xg(a-) red cells. XG transcription is tied to CD99 transcription, although the two related molecules do not associate on the cell surface.