AbstractThe sex chromosomes of many species differ in dosage but the total gene expression output is similar, a phenomenon referred to as dosage compensation. Previously, diverse mechanisms were postulated to account for compensation in distantly related taxa. However, two recent papers present evidence that dosage compensation in Drosophila, mammals and nematodes share the property that there is an approximately two‐fold upregulation of the single active X chromosome in each case.1,2 The results suggest that a common mechanism might operate in these different cases. BioEssays 28: 565–568, 2006. © 2006 Wiley Periodicals, Inc.