Schematic representation of the genomic organizations of XG and CD99 on the X and Y chromosomes and the location of SNP rs311103. The genomic organizations of the RefSeq transcripts of XG (accession NM_175569) and CD99 (NM_002414) in the National Center for Biotechnology Information database are represented schematically (based on assembly GRCh37). The blue rectangles represent the exon regions. The red dashed lines indicate the pseudoautosomal boundary (PBD). The genomic DNA sequences of the X chromosome 2240-2810-kb regions of 8 females and 8 males with different Xg^a/CD99 blood group phenotypes were determined in our pilot study.