Dimas et al. examined expression quantitative trait loci (eQTLs) in male and female cell lines from four HapMap populations; this is the first investigation of whether sex-specific eQTLs exist in humans. The authors found that 12–15% of eQTLs are sex-biased: they affect expression levels in only one sex. In addition, some eQTLs shared by both sexes have a substantially greater effect in one sex. This work shows that considering the sexes separately is likely to be beneficial in studies of the contribution of genetic variation to disease.