ESR1 and breast cancer: In this study, we utilized two such murine breast cancer cell lines: McNeuA, a HER2+ breast cancer cell line derived from a spontaneously arising mammary carcinoma in a MMTV-neu transgenic mouse [19], and Met-1, an estrogen receptor-negative (ER-) breast cancer cell line derived from a mammary carcinoma in a MMTV-PyMT transgenic mouse (FVB/N-Tg(MMTV-PyVmT) [18].