In one study of 43 postmenopausal women with ER+, progesterone receptor-positive (PgR+) breast cancer who received neoadjuvant tamoxifen or anastrozole for four months, the rate of clinical response [CR + PR (partial response)] was 64%, 31%, and 31% for patients with RS < 18, RS18–30, and RS ≥ 31, respectively [34]. This evidence concerns the gene ESR1 and breast cancer.