Overall, patients with ER+/HER2+ tumours have a poorer clinical response to tamoxifen than those with ER+/HER2− tumours, while they remain responsive to AIs (Ellis et al. 2001, 2003, 2006, Dixon et al. 2003, Zhu et al. 2004, Smith et al. 2005). Here, ESR1 is linked to neoplasm.