tokens: Oral - contraceptive use protected against ovarian cancer both for carriers of the BRCA1 mutation ( odds ratio , 0 . 5 ; 95 percent confidence interval , 0 . 3 to 0 . 9 ) and for carriers of the BRCA2 mutation ( odds ratio , 0 . 4 ; 95 percent confidence interval , 0 . 2 to 1 . 1 ) .