Scientists haven't worked out all the details of X inactivation, and a new study prolongs the puzzlement.
They speculated that Tsix might also control the random X inactivation of women.
Migeon speculates that Tsix controls imprinted, but not random, X inactivation. "Human placenta is not imprinted, so the fact that we don't have an effective Tsix molecule would make sense," she says.
The study looked at human cells that were far past the period of X inactivation, says Harvard researcher Jeannie T.
Migeon's finding may help explain why women don't have imprinted X inactivation, says Andrew Feinberg, a biologist from Johns Hopkins who wasn't involved in the study.
Scientists first recognized the phenomenon of X inactivation 4 decades ago.
Another new study offers an intriguing explanation for that finding and suggests a mechanism for how X inactivation occurs.
The new work doesn't come close to resolving all the questions surrounding X inactivation, but optimistic investigators contend that they're closing in on a better understanding of the puzzling event.
Ultimately, it'll look a lot more simple and elegant," says Neil Brockdorff, who studies X inactivation at the Imperial College School of Medicine and Hammersmith Hospital, both in London.
Lyon of the Medical Research Council's Mammalian Genetics Unit in Harwell, England, proposed in 1960 the idea of X inactivation, with the shutoff chromosome becoming the Barr body.
Although researchers are still not sure exactly why this occurs, many assert that X inactivation initially arose to prevent the genetic inequity that would result if females had a double dose of active X chromosome genes.
Willard at Stanford University and another led by Neil Brockdorff of the Medical Research Council Clinical Research Center in Harrow, England, have isolated two forms of a gene that may play a role in X inactivation in females.