ERBB4 and breast carcinoma: In BT474 cells, generally defined as a breast cancer cell type over-expressing ERBB2, we also detected expression of all four ERBB receptor members, and in this cell line combined knock down of ERBB1 and ERBB2 was sufficient to enhance ruxolitinib toxicity, though this effect was further enhanced by the additional knock down of ERBB4 (Figure 5B).