Expressed in erythroid tissues, including thymus, bone marrow and fetal liver, and in several nonerythroid tissues, such as heart, placenta, skeletal muscle, thyroid and trachea, as well as in skin fibroblasts. Expression is low or undetectable in other tissues. O-glycosylated. XG is responsible for the Xg blood group system. The gene coding for this protein is located in the pseudoautosomal region 1 (PAR1) of X and Y chromosomes. Belongs to the CD99 family. Blood group antigen gene mutation database