At the clinical level, Kaplan–Meier analysis of expression cohorts of breast tumor showed that high HDAC2 expression levels significantly (p-value < 0.001) correlate with poor relapse-free survival and poor overall survival [hazard ratio (HR) > 2] in tamoxifen or endocrine therapy-treated ER+ breast cancer patients (Figure 1C). This evidence concerns the gene ESR1 and breast cancer.