In 2009, researchers in Europe discovered that “shutting off” the FOXL2 gene in adult mice with the estrogen-blocking drug Tamoxifen turned females’ ovaries into testes within just three weeks.

The change was a thorough one; the altered organs were testes right down to the structure of their cells and their portfolio of active genes. They developed testosterone-secreting Leydig cells, which pumped out as much of the hormone as their counterparts in XY mice. They only fell short of actually producing sperm.

— Ed Yong, Not Exactly Rocket Science (Dec 2009)

The authors of the study believe this discovery could have important medical implications, but neglected to identify transgender men as potential beneficiaries. If the science applies to humans and the FOXL2 gene can disabled, this could provide an alternative to testosterone replacement for transgender men: gene therapy to turn ovaries into testosterone-producing testes.