A synaptobrevin-like gene in the Xq28 pseudoautosomal region undergoes X inactivation

The X and Y chromosomes that maintain human dimorphism are thought to have descended from a single progenitor, with the Y chromosome becoming largely depleted of genes. A number of genes, however, retain copies on both X and Y chromosomes and escape the inactivation that affects most X-linked genes in somatic cells. Many of those genes are present in two pseudoautosomal regions (PARs) at the termini of the short (p)³ and long (q)^4,5 arms of the sex chromosomes. For both PARs, pairing facilitates the exchange of information, ensuring the homogenisation of X and Y chromosomal material in these regions. We report here a strikingly different regulation of expression of a gene in Xq PAR. Unlike all Xp PAR genes studied so far, a synaptobrevin-like gene, tentatively named SYBL1, undergoes X inactivation. In addition, it is also inactive on the Y chromosome, thereby maintaining dosage compensation in an unprecedented way.