Comparative genome analysis between two distantly related species allows the organization of genes to be traced from a common ancestor. When several genes are mapped in one species and these genes are then localized in the distantly related species, then the genomic content of this region can be inferred in the common ancestor. If two species are closely related in evolutionary terms, then larger blocks of the genome will be conserved. Therefore these segments can be easily traced back to a common ancestor. Such comparisons have made it possible to trace the evolutionary origins of regions of the mammalian X chromosome (1).