Interestingly, while the number of ER-positive tumors was not significantly different in both cohorts, the number of ER-negative breast cancers was sharply reduced in the erlotinib-treated cohort (n = 5 versus n = 19, respectively), indicating that erlotinib was effective in preventing the emergence of ER-negative, but not ER-positive, breast cancers in this mouse model (Table 1). This evidence concerns the gene ESR1 and breast cancer.