In order to test the validity of canine mammary cancer as a model for human breast cancer from a hormonal point of view, the aims of this study were to compare in HBC and CMC samples the immunohistochemical expression of Arom and several hormone receptors (ERα, ERβ, PR, and AR), to analyze the intratumor steroid hormone concentrations (17β-estradiol, estrone sulfate, progesterone, androstenedione, dehydroepiandrosterone, and testosterone), and to study possible associations among these markers. The gene discussed is ESR1; the disease is breast carcinoma.