A gene that silences the X chromosome.

Counting to two doesn't sound like an impressive feat, but it's a matter of life and death for women's cells.

The genetic activity of a single X chromosome is sufficient for a human cell, but women must somehow deal with a double dose. Fortunately, mammals have evolved a way to silence one of the two X chromosomes in female cells.

More than 30 years after researchers discovered this phenomenon, however, the mechanisms by which cells count their X chromosomes and inactivate unneeded ones remain elusive.

"People have really scratched their heads over this," says Alan Ashworth of the Institute of Cancer Research in London.

Two new reports add to a growing body of evidence that an unusual gene called Xist may hold the key to X chromosome inactivation. While most genes encode proteins, Xist produces a short strand of RNA, a nucleic acid similar to DNA.

Several clues point to Xist's importance in silencing one copy of a female's X chromosome. Xist is active on the inactivated X chromosome but inactive on the active copy.

Furthermore, the RNA strands made by Xist cover the inactivated X chromosome "like a sheath," says Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass.

Jaenisch and other scientists have also shown that mutations in Xist prevent inactivation of the X chromosome on which it resides. In recent experiments, he and his coworkers added copies of a small X chromosome fragment that includes Xist to another chromosome of a male mouse embryo cell.

This technique tricks the male cell, which apparently calculates that it contains more than one X chromosome, into activating the Xist gene and turning off a nearby gene that the researchers had added along with Xist, Jaenisch's group reported in the July 12, 1996 Cell.

Jaenisch and Whitehead colleague Jeannie T. Lee now report in the March 20 Nature that the X chromosome material they add seems to have the ability to shut down an entire chromosome.

When the pair examined the activity of four genes spread along chromosome 12, to whose tip they had added the X chromosome fragments, they found all four genes silent. The researchers also observed that the RNA strands made by the Xist genes were spread along the entire length of the chromosome.

The X chromosome fragment added by Jaenisch and Lee is big enough to hold several genes, leaving unsettled the question of whether Xist acts alone.

Ashworth and his colleagues have started to address that issue by working with a smaller piece of the X chromosome, containing Xist and little else. Like Jaenisch's group, Ashworth's team has added this genetic material to non-X chromosomes in male mouse cells.

Once again, the male cells seem to think they contain more than a single X chromosome. They activate the added Xist genes and sometimes even the Xist gene on their one X chromosome.

This result, also published in the March 20 Nature, strongly implies that cells tally their X chromosomes by simply counting Xist genes, says Ashworth.

While this new research has begun to reveal the secrets behind X chromosome inactivation, scientists must still explain how a cell counts Xist genes and how the Xist RNA silences the thousands of genes on an X chromosome.

Answering the latter question may help explain how other animals manage their sex-related chromosomal differences. In fruit flies, for example, females do not turn off an X chromosome, and males hyperactivate their single copy so that it does the job of two.

While that action seems the opposite of inactivating a chromosome, some recent research hints that fruit flies, like mammals, use RNA to change the X chromosome's activity, observes geneticist Helen K. Salz of Case Western Reserve University School of Medicine in Cleveland.