Genes evolve at different rates depending on the strength of selective pressure 
to maintain their function. Chromosomal position can also have an influence [1] 
[2]. The pseudoautosomal region (PAR) of mammalian sex chromosomes is a small 
region of sequence identity that is the site of an obligatory pairing and 
recombination event between the X and Y chromosomes during male meiosis [3] [4] 
[5] [6]. During female meiosis, X chromosomes can pair and recombine along their 
entire length. Recombination in the PAR is therefore approximately 10 times 
greater in male meiosis compared with female meiosis [4] [5] [6]. The gene Fxy 
(also known as MID1 [7]) spans the pseudoautosomal boundary (PAB) in the 
laboratory mouse (Mus musculus domesticus, C57BL/6) such that the 5' three exons 
of the gene are located on the X chromosome but the seven exons encoding the 
carboxy-terminal two-thirds of the protein are located within the PAR and are 
therefore present on both the X and Y chromosomes [8]. In humans [7] [9], the 
rat, and the wild mouse species Mus spretus, the gene is entirely X-unique. 
Here, we report that the rate of sequence divergence of the 3' end of the Fxy 
gene is much higher (estimated at 170-fold higher for synonymous sites) when 
pseudoautosomal (present on both the X and Y chromosomes) than when X-unique. 
Thus, chromosomal position can directly affect the rate of evolution of a gene. 
This finding also provides support for the suggestion that regions of the genome 
with a high recombination frequency, such as the PAR, may have an intrinsically 
elevated rate of sequence divergence.