ERBB2 and breast cancer: To develop a mouse model of breast cancer, it is necessary to have experimental cells, and patient derived primary cells such as MCF-7 (ER+, PR+, HER2-), MDA-MD-231 (ER+, PR+, HER2-), MC4-L2 (ER+), E0771 (ER+) and 4T1 (ER-) (Fig. 1).