Research has discovered more about how male versus female brain differentiation occurs. They altered female rat brains by giving estradiol, a testosterone derivative that triggers a mechanism by which certain genes in the brain are "unsilenced," allowing them to initiate the process of masculinization. This process involves a group of enzymes known as DNA methyltransferases or Dnmts that modify DNA to repress gene expression. Physically, these were female rats, but in their reproductive behavior, they were males. (Bridget M Nugent and Margaret M McCarthy, et al. “Brain feminization requires active repression of masculinization via DNA methylation.” Nature Neuroscience, 2015; DOI: 10.1038/nn.3988)