ERBB2 and neoplasm: According to HER2DX, ERBB2 mRNA-low tumours should be considered clinically HER2-negative since the distinction between the HER2DX ERBB2 mRNA-low versus ERBB2 mRNA-medium categories is based on an optimal cut-off to identify clinically HER2-positive tumours from HER2-negative tumours, according to the ASCO/CAP HER2 definition.21