ESR1 and breast cancer: Tumor xenografts were generated from two breast cancer cell lines, which were selected as representatives of different breast cancer types: AU565 exemplifies HER2 positive breast cancer, which overexpresses epidermal growth factor 2 (HER2) receptor and lacks both estrogen (ER) and progesterone (PR) receptors and MDA-MB-231 illustrates triple-negative breast cancer lacking HER2, ER and PR receptors.