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) and long (q) 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.