In order to examine the potential prognostic and treatment-predictive values of stromal ERK phosphorylation we therefore analyzed a unique randomized trial including 564 pre-menopausal breast cancer patients randomized to 2 years of tamoxifen or no adjuvant treatment after surgery, as well as a second cohort of 179 pre- and post-menopausal patients and focused on CAFs. Here, MAPK1 is linked to breast cancer.