The MMTV-Neu (NEU) tumor was ER–/PR–/HER2+, while the MMTV-PyMT (FF99WT) tumor was ER–/PR–/HER2low; the BRCA1-null (BRCA1) tumor which mimics basal-like breast cancers was ER–/PR–/HER–, and the 4T1 transplant tumor was a known ER–/PR–/HER– mouse model. This evidence concerns the gene ESR1 and breast cancer.