Epidemiological studies in women strongly correlate a reduced risk for developing breast cancer with early full-term pregnancy. While the molecular mechanisms that give rise to pregnancy-protective effects are not known, several hypotheses have been proposed. The paper presented here addresses the theory that the hormonal cues delivered during early pregnancy promote long-term biochemical changes in the mammary epithelia that influence the growth potential of cells when exposed to carcinogenic insult. Specifically, the authors are interested in the possibility that the tumor suppressor protein, p53, may function as such a biochemical mediator. Here, studies explore the spatial and temporal expression patterns of p53 in vivo using combinatorial hormone treatment to mimick early pregnancy in both rat and mouse models.