The mouse Y chromosome plays a fundamental role in the control of primary sex determination and fertility. Both genetic and molecular biological evidence has shown that much of the necessary information is contained in a minute piece of the Y (the Sxr region) which has arisen by a duplication of the pericentric region of the normal Y and the transposition of one copy to the distal pseudoautosomal region. The present article describes the isolation of random Y-chromosome probes and their use to investigate this Sxr region at the molecular level. Total mouse Y-chromosome libraries were constructed from flowsorted material and a Sxr regional library after specific microdissection and cloning. Transcription has been detected in the testis using both Sxr-specific and non Sxr-located genomic probes taken from these libraries. In addition, we have been able to confirm the presence of an active steroid sulphatase gene on the mouse Y. This gene is located in the distal portion of the pseudoautosomal region and is tightly linked to Sxr. Finally, using an Sxr-specific probe we can define multiple Y-chromosome haplotypes in the mouse showing that the region is evolving very rapidly.

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