Methods: We will randomly sample from the KPCO population of approximately 50,000 women ages 55-69 who have recently had a negative screening mammogram. An invitation packet (letter, information pamphlet, consent form, eligibility questionnaire, and opt-out postcard) will be sent in batches of 100, up to a maximum of 500 women. The consenting women will receive a second packet with a cancer risk-factor questionnaire, a link to the on-line Block brief food frequency questionnaire, and a specimen collection kit (with illustrated instructions) for collecting a fecal and urine specimen. Participants (minimum 60, maximum 80) will ship the specimens to the DCEG repository. As amended, consecutive newly diagnosed breast cancer cases (target N=60) will be enrolled in the oncology clinic prior to definitive surgery, with the same eligibility criteria as for the random sample. For contemporaneous controls, participants in the randomly sampled, mammogram-negative population will be recruited back (target N=60). Urine estrogens will be quantified by DCEG at NCI Frederick. Fecal microbial classification and enzymatic expression and activity will be performed at the University of Maryland Medical School (UMMS).