She was slow to reveal her secrets, but the X chromosome has now bared it all. Researchers said yesterday they have determined with 99.99 percent accuracy the genetic code of the X chromosome, which lies at the core of human femaleness.

The newly completed picture of the X -- one of the last orders of business for the human genome project -- falls far short of explaining all the mysteries of what makes a woman.

But by determining the exact order of virtually all 155 million "letters" of code on the X -- the counterpart to the male Y chromosome -- scientists have confirmed how sex evolved and are much closer to explaining some of the differences between men and women.

The X's unveiling also brings into focus the molecular underpinnings of hundreds of genetic diseases, far more than have been discovered on any other human chromosome. And it appears to have revealed a long-sought role for much of the body's "junk DNA," which is especially prevalent on the X, and whose lack of apparent function has long baffled scientists.