Canine mammary cancer shares many common features with human breast cancer, including the age of onset, hormonal etiology, stage, tumor diversity, lymph node metastasis, and genetic abnormalities including breast cancer susceptibility gene 2 (BRCA), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), and TP53 mutations, and protein expression, including human epidermal growth factor receptor 2 (HER2), estrogen receptor, progesterone receptor, and p63 (2–6). Here, ESR1 is linked to breast cancer.