ESR1 and breast cancer: With longer follow‐up of an open‐label, single‐arm, Japanese phase 2 study conducted to investigate the efficacy and safety of palbociclib plus letrozole as first‐line treatment in postmenopausal patients with estrogen receptor‐positive/human epidermal growth factor receptor 2‐negative advanced breast cancer, median progression‐free survival was 35.7 months (95% CI, 21.7‐46.7).