ESR1 and breast cancer: Patients with high ERK phosphorylation in CAFs showed a significant improved recurrence-free survival upon tamoxifen (Hazard Ratio [HR]: 0.471, 95% Confidence Interval [CI]: 0.263 to 0.843, Univariate Cox regression: P = 0.011) (Figure 2C) similar to the response seen for all ERα-positive breast cancer patients (Hazard Ratio [HR]: 0.620, 95% Confidence Interval [CI]: 0.441 to 0.871, Univariate Cox regression: P = 0.006) (Figure S2B).