While ESR1 alterations, such as amplifications, can be identified in up to 30% of ER+ BC patients [27,28], it is still uncertain whether this alteration has clinical significance in terms of ET resistance: while some studies have found that ESR1 amplifications were associated with improved disease-free survival [29,30] several others studies report an association between ESR1 amplifications and tamoxifen resistance [31,32]. Here, ESR1 is linked to breast cancer.