ESR1 and breast cancer: Since tamoxifen induces cell growth arrest and apoptosis in ER + breast cancer cells by phosphorylating JNK and p38 [26], we examined the levels of phosphorylated p38 (p-p38) and JNK (p-JNK), as well as cleaved poly-ADP-ribose polymerase (PARP), in tamoxifen-treated cells.