The remarkable findings refute the generally held view that sex is determined at birth and is irreversible in later life.
They could also lead to treatments for certain gender disorders in children, early menopause in women and even eventually non-surgical sex changes.
In mammals, males have XY chromosomes and females XX. The new research shows that another gene is responsible for switching women into men.
If the FOXL2 is switched on then the body grows ovaries, switched off and they are replaced by testicles.
But what really surprised the researchers is that the process continues after birth and the body remains in a constant tussle to either switch on or off the gene - even in adulthood.
"No one would have betted on this," said Professor Mathias Treier of the University of Cologne in Germany. "That's why the finding is so spectacular,"
The team found that in adult mice by turning off the FoxL2 gene, the ovary cells started to change testicular cells. This suggest the same effect could happen to humans.
"It was thought that you were born either female or male and then your body forgot about it," said Dr Robin Lovell-Badge, the co-author at Britain's Medical Research Council's National Institute of Medical Research.
"But this suggests that the battle of the sexes continues all the way through your life, "
The researchers believe that the process could be similar to that in some fish who are able switch sexes at times when there is a scarcity of either males or females.
I found this pretty interesting. I wonder what'll come from it.