The mouse Fxy gene was translocated into the highly recombining pseudoautosomal 
region comparatively recently in evolutionary terms. This event resulted in a 
rapid increase of GC content. We investigated the consequences of the 
translocation further by sequencing exons and introns of Fxy in various rodent 
species. We found that the DNA fragment newly located in a highly recombining 
context has acquired every property of a GC-rich isochore, namely increased GC 
content (especially at the third codon positions of exons), shorter introns and 
high density of minisatellites. These results strongly suggest that 
recombination is the primary determinant of the isochore organization of 
mammalian genomes.