The theory that race is merely a social construct has been challenged by the findings of researchers at the Stanford University School of Medicine, published in the American Journal of Human Genetics as "Genetic Structure, Self-Identified Race/Ethnicity, and Confounding in Case-Control Association Studies". One of the researchers, Neil Risch, noted: "we looked at the correlation between genetic structure [based on microsatellite markers] versus self-description, we found 99.9% concordance between the two. We actually had a higher discordance rate between self-reported sex and markers on the X chromosome! So you could argue that sex is also a problematic category. And there are differences between sex and gender; self-identification may not be correlated with biology perfectly. And there is sexism."