To extend our observation to other models of ET-resistant breast cancer, we used the ER+ fulvestrant-sensitive (HCI-011) patient-derived xenograft organoid (PDxO) model, and its fulvestrant-resistant counterpart (HCI-011-FR), which was derived by allowing a fulvestrant responsive xenograft tumor to recur over a long-term fulvestrant treatment [53]. This evidence concerns the gene ESR1 and breast cancer.