A literature search was performed in PubMed (http://www.ncbinlm.nih.gov/pubmed) using the terms “canine mammary tumor biomarkers”, “human breast cancer biomarkers”, “Ki-67”, “PCNA”, “p53”, “E-cadherin”, “CEA”, “CA 15-3”, “VEGF” EGFR”, “HER-2”, “ER”, “PR”, “COX-2”, “BRCA 1”, “BRCA 2”, “miRNA”, “CSCs” and “CSCs”. This evidence concerns the gene ESR1 and breast cancer.