ESR1 and breast cancer: Until recently, however, the classification of canine mammary tumors did not consider the prognostic elements that are contemplated in the classification of human breast cancer: the concurrent expression of the pivotal cancer-related biomarkers such as ERα, PR, HER2, Ki-67, and basal markers have thus been evaluated only in a very few studies, dealing either with small cohorts or without complete follow-up [17, 22, 27].