WASHINGTON, D.C.: TIME's Christine Gorman reports that the discovery of a strong association between ovarian cancer and mutation of the p53 gene still leaves the mystery surrounding this ruthless killer's cause unresolved. Given that one- third of ovarian cancer cases are unrelated to the gene, "it's still quite possible to a have a p53 mutation and not get the disease," Gorman notes. "There's an association, but you can't just say 'if you have the mutation, you're going to get ovarian cancer'. Trotting every fifty-year-old woman in for a genetic test would scare an awful lot of people unjustifiably." Still, the finding published today in the Journal of the National Cancer Institute establishes the first concrete risk factor for the disease. With fewer ovulations come fewer mutations of the p53 gene and a substantially reduced risk for developing the cancer that kills 15,000 women in the U.S. each year. But while pregnancies and birth control pills, which lower ovulation histories, appear to provide the surest means of slashing the risk for developing ovarian cancer, such defenses are far from a practical weapon. Without an effective screening test, ovarian cancer looks set to remain a brutally efficient killer.