Chemical Carcinogen-Induced Tumorigenesis in Parous, Involuted Mouse Mammary Glands

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Abstract
Breast cancer remains the major cancer among women in the United States in terms of noncutaneous cancer incidence and the second leading cause of cancer deaths ( 1 ). Among the many risk factors for breast cancer, reproductive history, genetic background, and age are the strongest and most consistent ( 2 , 3 ). However, the strongest protective factor is also related to reproductive history, i.e., early age at first pregnancy (⩽20 years of age), which confers a 50% reduction in lifetime risk compared with the lifetime risk of breast cancer in nulliparous women. The protective effect of early first pregnancy has been demonstrated repeatedly in numerous epidemiologic studies and provides a physiologically operative model to achieve practical and affordable prevention of breast cancer in humans.