Treating sex as a biological variable means planning and reporting studies in a way that results are informative for both sexes. Preclinical work historically relied on single sex designs, which can conceal differences in disease mechanisms and treatment response. Funders now expect researchers to factor sex into design, analysis, and reporting, a shift exemplified by the NIH policy introduced to balance sex in in vitro, ex vivo and in vivo studies.

In practice, it is highly advisable to state the sex of animals, justify any single sex choice, power studies to detect sex effects when relevant, and analyse or stratify by sex. ARRIVE 2.0 provides clear reporting guidance that supports reproducibility and transparent methods. For further guidance and examples, see ARRIVE 2.0 and our Good Research Practice training course.