In the December 2016 issue, the piece “Sex on the brain: Unraveling the differences between women and men in neurodegenerative disease” (Nat. Med. 22, 1370–1372, 2016), the explanation of the US National Institutes of Health's (NIH's) policy on sex differences in animal research is incorrect. The policy does not require that researchers include equal numbers of male and female animals, but rather that they account for sex as a biological variable in their research design. The error has been corrected in the HTML and PDF versions of the article.