Sex in mammals is genetically determined. In humans, females have two X chromosomes, while males have one X and one Y chromosome. However, some individuals are born with male genitalia despite having two X chromosomes, a condition known as XX male sex reversal.

A team of researchers, led by Paul Thomas, University of Adelaide, Australia, has now determined that overexpression of the Sox3 gene in mice causes frequent XX male sex reversal. The clinical relevance of this was highlighted by the discovery of genomic rearrangements in the regulatory region of the human SOX3 gene in three patients with XX male sex reversal. The authors therefore conclude that SOX3 genomic rearrangements are likely to be a significant cause of XX male sex reversal.