Experimental schema. Experimental schema. Female C3 (1)/SV40 Tag mice were weaned from their mothers at 3 wk of age; behavior was assessed using a standard open-field test (see Materials and Methods). Based on the results of this initial open-field testing, the mice were divided into two cohorts with a balanced distribution of behavior: cohort A, group-housed cohort (four mice per cage); cohort B, individually housed cohort. At 13 wk of age, behavior testing was repeated. At 15 wk of age, global gene expression in the mammary glands of a subset of isolated versus group-housed mice was determined. Beginning at 16 wk, mammary gland tumor volumes were measured weekly in both cohorts. At 20 wk, gene expression was evaluated in a second subset of mice, and at 22 wk serum corticosterone reactivity in response to a mild, acute stressor was measured. Mice were then sacrificed and mammary glands were weighed and the pathology was examined. J. Bradley Williams et al. Cancer Prev Res 2009;2:850-861 ©2009 by American Association for Cancer Research