The aim of this study is to investigate the association between seven single nucleotide polymorphisms (SNPs) in canine ESR1 gene (rs397512133, rs397510462, rs851327560, rs397510612, rs852887655, rs852684753 and rs852398698) and clinicopathological features of canine mammary tumors and the clinical outcome of the disease. Here, ESR1 is linked to breast cancer.