While it has already been recognized that primary breast cancers rarely harbor ESR1 mutations (Jeselsohn et al., 2014; Toy et al., 2013), most studies thus far have evaluated patients who had been pretreated with palliative endocrine therapy, suggesting that these mutations become enriched during treatment with AIs (Schiavon et al., 2015). This evidence concerns the gene ESR1 and breast carcinoma.