ESR1 and breast adenocarcinoma: We used three complementary in vivo mice models of breast cancer: transgenic polyoma middle T oncogene-induced (MMTV-PyMT) mice (FVB/N genetic background), mice implanted with the human ER+ breast adenocarcinoma MCF7 cell line or with a hormone-dependent (ER+) breast PDX tumor (Figure 1A).