ESR1 and breast carcinoma: We previously identified p38 MAPK activation as a mechanism responsible for PTK6 shRNA-induced apoptosis of HER2-targeted therapy-resistant breast cancer cells.25 To determine whether this mechanism of survival regulation by PTK6 might be conserved in ER+ breast cancer cells, we assessed p38 activity in tamoxifen-resistant ER+ breast cancer cells expressing PTK6 shRNA.