Erlotinib as a chemopreventative agent in (MMTV-Cre) BRCA1 flox/flox p53+/- mice. (A) Erlotinib prevents the emergence of estrogen receptor-negative (ER-) but not estrogen receptor-positive (ER+) breast cancers in mouse mammary tumor virus-Cre recombinase (MMTV-Cre) BRCA1 flox/floxp53+/- mice. Virgin female mice were treated as controls or with 100 mg/kg erlotinib via oral gavage once daily, seven days per week. Tumors were recorded when they were first palpated. Kaplan-Meier graphing and analysis of disease-free survival were performed using the GraphPad Prism software package. (B) The growth of established breast cancers is not affected by erlotinib treatment. Mice that developed tumors in the control cohort were switched to erlotinib treatment, and the tumor volume relative to the tumor volume at diagnosis was plotted against treatment time. ER status was determined after necropsy. The trend line for vehicle control-treated tumors was established on the basis of the tumor volumes of control mice.