Similarities of breast cancer between humans and dogs have also been identified at the molecular level, involving the overexpression of steroid hormone receptors, such as estrogen (ER) and progesterone (PR), proliferation markers (Ki67, AgNOR), epidermal growth factor (EGF), and cyclooxygenases, among others [8,16,17,18,19,20,21]. This evidence concerns the gene ESR1 and breast carcinoma.