From Scientific American:
Female hormones circulating in the brain determine masculine behavior, at least in mice. Estrogen–the quintessential female hormone responsible for regulating the reproductive cycle–turns lady mice into wannabe male mice when it is allowed to penetrate the brain during development, according to new research.
Neuroscientist Julie Bakker of the University of Liege in Belgium and her colleagues proved this in the course of solving one of the longstanding riddles of brain development. Although it had long been known that a certain protein–alpha-fetoprotein (AFP)–plays a key role in mouse brain development by binding to estrogen, it was unclear whether AFP facilitates the development of female brains by carrying the hormone or simply by blocking it from entering the brain.
More here.