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 genes1. 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 cells2. Many of those genes are present in two pseudoautosomal regions (PARs) at the termini of the short (p)3and long (q)4,5arms of the sex chromosomes. For both PARs, pairing facilitates the .exchange of information, ensuring the homogeni-sation of X and Y chromosomal material in these regions6–10. 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.