ERBB2 and neoplasm: We used ER rather than ER and/or PR to define an endocrine sensitive tumour because ER status was thought to be the only recorded factor importantly predictive of the proportional reductions from endocrine therapy.[26]Knowledge of HER2 status would permit further classification of ER-positive tumours as luminal-A (HER2-negative) or luminal-B (HER2-positive).