Transgenic mouse lines are available for studies of spontaneously occurring breast tumors of different major subclasses: MMTV-Neu transgenic mouse line develops HER-2 positive breast cancer, MMTV-HRAS transgenic mouse line develops oncogene (HRAS)-driven breast cancer, and MMTV-PyMT mouse line develops ER+ breast cancer initially which then transforms into ER-negative breast cancer [26]. Here, ESR1 is linked to breast cancer.