E-Mail BOSTON - Researchers at Massachusetts General Hospital (MGH) have uncovered new clues that add to the growing understanding of how female mammals, including humans, "silence" one X chromosome. Their new study, published in Molecular Cell, demonstrates how certain proteins alter the "architecture" of the X chromosome, which contributes to its inactivation. Better understanding of X chromosome inactivation could help scientists figure out how to reverse the process, potentially leading to cures for devastating genetic disorders. Female mammals have two copies of the X chromosome in all of their cells. Each X chromosome contains many genes, but only one of the pair can be active; if both X chromosomes expressed genes, the cell couldn't survive. To prevent both X chromosomes from being active, female mammals have a mechanism that inactivates one of them during development. X chromosome inactivation is orchestrated by a noncoding form of RNA called Xist, which silences genes by spreading across the chromosome, recruiting other proteins (such as Polycomb repressive complexes) to complete the task.