One of the first targeted agents in the treatment of cancer was the selectiveoestrogen receptor modulator (SERM) tamoxifen, which is an effective treatment forER+ breast cancers (3) because itcan mimic oestrogen and bind to the LBD pocket of the ER, but unlike oestrogen, italters the structure and function of ER so that this transcription factor is no longercapable of regulating gene expression (4). This evidence concerns the gene ESR1 and breast cancer.