We had used the FVB/N mouse mammary tumor virus-polyoma middle tumor antigen (MMTV-PyMT) model, which represents the “luminal B” subtype of human breast cancer based on gene expression patterns in relation to human breast cancers but initiates as an estrogen receptor–positive (ER+) tumor but eventually progresses into ER-negative tumor (5, 6). Here, ESR1 is linked to breast cancer.