ERBB2 and neoplasm: The fact that increased HER2 expression in recurrent tumours compared to primary tumours is relatively common strongly supports the need to retest for HER2 expression at relapse (whether local recurrence or distant metastases), particularly for patients with primary ER+/PR-tumours as well as those with, low proliferation indexes and late recurrences, in order to select the best candidates for new anti-HER2 ADC therapies.