Starting in October, biomedical researchers will have to counterbalance their use of male animals and cells with the equivalent in female biological tissues and test subjects. At least thats what theyll have to do if they want to get funding from the US National Institutes of Health (NIH), the largest source of medical-research funding in the world. This change, NIH researchers announced in Nature today, will help close the sex gap that continues to exist in preclinical research — the research stage in which drugs and medical interventions are tested on animals and biological tissues — and will \ensure that the health of the United States is being served by supporting science that meets the highest standards of rigor.\ But preclinical research continues to include a majority of male animals and tissues.