Normal development of the female rat brain requires active silencing of key male-associated genes during a critical period just before and after birth.

Bridget Nugent at the University of Maryland School of Medicine in Baltimore and her colleagues studied a region of the rat brain that differs significantly in males and females. They found that in females shortly after birth, there were more methyl groups attached to DNA in this brain region, reducing gene expression.

When they injected newborn females with a male hormone, the activity of an enzyme that adds methyl groups to DNA decreased. When they blocked the activity of this enzyme in female rats during the critical period after birth, the animals showed male-like gene expression and sexual behaviour.

Nature Neurosci. http://dx.doi.org/10.1038/nn.3988 (2015)