Flip through a few TV channels or browse the Internet for a bit and you will be quickly reminded that, in our day and age, everyone is thinking about ‘sex’. Biologists think about sex too – albeit more in the biological sense than the act itself. The problem is they don't think enough about it. Indeed, though most animals display marked differences in sexual anatomy and reproductive function, sex is regularly overlooked in biomedical research at both the clinical and basic science levels 1 . Over 25 years ago, The National Institutes of Health recognized this as problematic; exclusion of women from large clinical trials blunted their ability to detect sex differences in the safety and efficacy of therapeutic drugs. In 2001, the Institute of Medicine echoed these concerns, calling for expansion of research into sex differences at the biochemical and cellular levels 2 . Despite this, investigators still regularly ignore the sex of cell lines studied in vitro 2 , as well as failing to include both sexes in animal studies. In this article, we briefly discuss the nature of sex differences and highlight their importance to future basic and translational research.