During prenatal development there is a restricted time frame during which the brain takes on male or female characteristics. Scientists had thought that once this window closed, it could not be reopened but researchers have found otherwise. DNA methyltransferases or Dnmt enzymes control expression of genes that play a role in inflammation and immunity, and also in the sexual differentiation of the brain. Microglia, inflammatory immune cells, also appear to play a role in masculinization, in part through their production of prostaglandins, a neurochemical normally associated with illness. The immune system is integral to the development of the brain but this study is the first to show that it is also important for establishment of sex differences in the brain. (Margaret McCarthy, PhD, and Bridget Nugent, PhD, et al. “Brain feminization requires active repression of masculinization via DNA methylation.” Nature Neuroscience, 2015; DOI: 10.1038/nn.3988)