ESR1 and breast cancer: In a study on immunohistochemical detection of ERα in a canine mammary tumor [4], it was reported that ERα immunostaining was localized in the nuclei of normal, benign, and malignant epithelial and also in myoepithelial cells, whereas the normal and neoplastic stromal cells, cartilaginous cells, and bone cells in mixed neoplasms were negative.