<h4>Background</h4>An open-label, single-arm, multicenter Japanese phase 2 study (J-Ph2) found first-line palbociclib plus letrozole to be effective and tolerable in postmenopausal Japanese women with estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER+/HER2-) advanced breast cancer (ABC), but overall survival (OS) data were immature. This evidence concerns the gene ESR1 and breast cancer.