Paradoxically, one of the strongest and extensively documented protective factors for breast cancer in both humans and rodent models is short-term stimulation with hormone, either by means of an early full-term pregnancy or with the hormones estrogen and progesterone. e protective properties of estrogen and progesterone were initially shown in 1962 by Charles Huggins  and the extensive studies in this area have been reviewed [5,6]. Studies over the past 30 years to document and understand the mechanistic basis for this phenomenon have used the traditional rat and mouse chemical-carcinogen-induced mammary tumor models. ese models use etiological agents that are thought not to be risk factors in human breast cancer. One of the known risk factors in human breast cancer is exposure to radiation . Ironically, in the only study done in the rat where radiation was the initiating agent, hormones were not protective . ese considerations raise the question of the relevance of the traditional models for understanding the mechanism of the protective eects of hormones.