Many studies have described mechanisms of tamoxifen resistance in breast cancer patients, such as the upregulation of ER transcriptional co-activators [5] or the expression of ER isoforms [6], the activation of several tyrosine kinase pathways such as PI3K/MAPK [7], or the dysregulation of tamoxifen metabolizing enzymes [8]. Here, ESR1 is linked to breast cancer.