The pseudoautosomal regions (PAR1 and PAR2) of the human X and Y chromosomes 
pair and recombine during meiosis. Thus genes in this region are not inherited 
in a strictly sex-linked fashion. PAR1 is located at the terminal region of the 
short arms and PAR2 at the tips of the long arms of these chromosomes. To date, 
24 genes have been assigned to the PAR1 region. Half of these have a known 
function. In contrast, so far only 4 genes have been discovered in the PAR2 
region. Deletion of the PAR1 region results in failure of pairing and male 
sterility. The gene SHOX (short stature homeobox-containing) resides in PAR1. 
SHOX haploinsufficiency contributes to certain features in Turner syndrome as 
well as the characteristics of Leri-Weill dyschondrosteosis. Only two of the 
human PAR1 genes have mouse homologues. These do not, however, reside in the 
mouse PAR1 region but are autosomal. The PAR regions seem to be relics of 
differential additions, losses, rearrangements and degradation of the X and Y 
chromosome in different mammalian lineages. Marsupials have three homologues of 
human PAR1 genes in their autosomes, although, in contrast to mouse, do not have 
a PAR region at all. The disappearance of PAR from other species seems likely 
and this region will only be rescued by the addition of genes to both X and Y, 
as has occurred already in lemmings. The present review summarizes the current 
understanding of the evolution of PAR and provides up-to-date information about 
individual genes residing in this region.