ESR1 and neoplasm: In a number of studies, both ER+ and ER− tumours were included in the analysis (Wright et al. 1992, Archer et al. 1995, Hayes et al. 2001, Lipton et al. 2002, 2003), therefore much of the reported resistance of HER2-positive tumours to endocrine therapy could be due to the ER-negative nature of the tumour rather than HER2-positivity per se (Wright et al. 1992, Dowsett et al. 2001).