To the Editor A recent Research Letter by Dr Burton and colleagues revealed that almost two-thirds of the trials included in ClinicalTrials.gov reported applying gender-based eligibility between 2017 and 2022 when, in fact, sex-based eligibility was used instead. According to the National Institutes of Health, sex is considered a biological component, including chromosomes and gonads, and gender comprises social, cultural, and behavioral factors that influence a person’s self-identity. Sex and gender can affect health in different ways and are not interchangeable.