‘Switch’ That Turns Off Excess Genes Identified

After a 30-year search, scientists have apparently identified the switch that keeps human female embryos from overdosing on their own genes. The Xist gene turns off excess genes permanently, apparently by covering them with a chemical blanket, a team from the Institute of Cancer Research in London reports in Nature. The discovery could some day allow researchers to turn off cancer and other undesirable genes, they said.

The excess genes are on the X chromosome, one of the microscopic strands that carry DNA. Men inherit one copy of that chromosome, women get two. Two is too many, because the resulting double dose of genes would kill an embryo. So in every cell of a female embryo, one copy of the X chromosome is shut off, except for certain genes. It is then inherited in this largely inert form by later generations of cells.