Sporadic breast cancer is a fatal disease most frequently diagnosed in American women from all ethnic groups, suggesting that primary prevention should be the ultimate goal for breast cancer control. We have developed a novel paradigm for breast cancer prevention arising from the well-established knowledge that an early first full-term pregnancy protects the breast against neoplastic transformation, as well as from our studies of the biological principle underlying this protection. We have shown experimentally that the first pregnancy induces the expression of a specific genomic signature in the breast that results from the completion of a cycle in this organ's differentiation driven by the reproductive process. This signature, in turn, is a biomarker associated with a possible overall lifetime decrease in breast cancer risk. We have shown in an experimental model that a short treatment with human chorionic gonadotropin, a placental hormone secreted during pregnancy, induces the same genomic signature that occurs in pregnancy, inhibiting not only the initiation but also the progression of mammary carcinomas, and stopping the development of early lesions such as intraductal proliferations and carcinoma in situ. These observations indicate that human chorionic gonadotropin given for a very short period, only until this genomic signature is acquired, has significant potential as a chemopreventive agent, protecting the normal cell from becoming malignant. This is a novel concept which challenges the current knowledge that a chemopreventive agent needs to be given for a long period of time to suppress a metabolic pathway or abrogate the function of an organ.