ESR1 and neoplasm: Patients with tumours expressing ER are candidates for endocrine therapy (Billam et al., 2009; Selli et al., 2016), while patients with tumours lacking ER expression are unresponsive to endocrine therapy, as these therapies are molecularly targeted therapies that work by modulating or antagonising ER, thus inhibiting the growth of the tumour (Billam et al., 2009; Gucalp and Traina, 2011; Mast and Kuppusamy, 2018; Schiff and Osborne, 2005).