To understand how sex as a biological variable (SABV) research policies could better account for entanglement requires more than the input from biological sciences; it necessitates a joint scientific, political, and feminist response. This chapter analyzes the mandate put forth by the US National Institutes of Health (NIH) that requires consideration of SABV in animal research. Adopted in 2016, this policy has been a point of contention among feminist researchers. Advocates from women’s health research policy argue that it corrects a long-standing reliance on male animals in many research fields, while skeptics and critics have expressed concern that the policy encourages overly binary, reductionist, and even inaccurate approaches to the study of sex-related variation. Although these two positions appear to be at odds, there is considerable common ground between them, pointing to the possibility of a productive exchange and a move toward mandates for research that emphasize entanglements of sex and gender as well as an embrace of more complex ways of describing variation within and across women and men and female- and male-classified animals. Research institutions, such as the NIH, as well as advocates of women’s health research have the opportunity to move toward recognizing the value—for science, health, and equity—of approaching sex/gender variation in all bodies as irreducibly complex and dynamic.