ERBB2 and breast cancer: To determine whether our findings of how RON signals in estrogen receptor positive (ER+) T47D cells would extend to other types of breast cancer cells, we engineered SUM-159PT cells (a cell line that is “triple negative” for ER, progesterone receptor, and the human epidermal growth factor receptor HER2) for conditional expression of RON using the same doxycycline-regulated system.