ERBB2 and breast cancer: However, the fact that we overexpressed heregulin could not exclude possible detrimental effects on drug sensitivity; therefore, we utilized an additional mouse model based on the endogenous heregulin‐expressing HER2‐positive breast cancer cell line JIMT‐1.24 Consistent with that observed with the BT474‐HRG xenograft model, triple combination therapy induced tumor regression and had no effect on body weight in this model.