The spontaneous, naturally occurring canine mammary tumors share many features with human breast cancer such as the predominant malignant histological type being adenocarcinoma [1-3] and the expression of estrogen and progesterone receptors (ER/PR), and epidermal growth factor receptor 2 (HER2) in subsets of canine mammary carcinoma [4-7]. Here, PGR is linked to adenocarcinoma.