But the flawed genes don't always cause ovarian cancer. They may trigger the disease in one woman while sparing both her mother and grandmother, even though all of them had the same mutant gene in all their cells.
Ovarian cancer is the most frequent malignancy of the female reproductive organs.
It will strike an estimated 26,700 American women this year. Until recently, experts estimated that 5 percent to 10 percent of ovarian cancers were passed to daughters through genes inherited from their mothers or fathers. The first to be discovered, BRCA1, is thought to be responsible for about 6 percent of ovarian cancers. A companion gene, BRCA2, discovered just last December, has been found to be responsible for about the same proportion. Dr. Jeffrey A. Boyd of the University of Pennsylvania said mutations of BRCA2 appear to be responsible for at least 5 percent of ovarian cancers, even though most women who have that gene have no clear family history of ovarian cancer.