The pseudoautosomal boundary in man is defined by an Alu repeat sequence inserted on the Y chromosome
The Y chromosome, which in man determines the male sex (reviewed in ref. 1), is composed of two functionally distinct regions. The pseudoautosomal region is shared between the X and Y chromosome and is probably required for the correct segregation of the sex chromosomes during male meiosis2. The second region includes the sex-determining gene(s), the presence of which is necessary for the development of testes. The two regions have contrasting genetic properties: the pseudoautosomal region recom-bines between the X and Y chromosome3; the Y-specific region must avoid recombination otherwise the chromosomal basis of sex-determination breaks down1. The pseudoautosomal region is bounded at the distal end by the telomere and at the proximal end by X- and Y-specific DNA. We have found that the proximal boundary was formed by the insertion of an Alu sequence on the Y chromosome early in the primate lineage. Proximal to the Alu insertion there is a small region where similarity between the X and Y chromosomes is reduced and which is no longer subject to recombination.