Lambda G28, a mouse genomic clone homologous to the human P3 gene and associated with a CpG island, also hybridizes to human probes for the neighboring GdX gene. The two genes, P3 and GdX (DXS253E and DXS254E), physically linked on the human X chromosome, lie within a similar physical distance on the mouse X chromosome. The CpG island corresponds to that at the 5' of the human GdX gene. The relative orientation of the two genes is the same. The DNA sequence in coding and noncoding regions is very conserved.