ERBB2 and neoplasm: These variables might trouble the testing sensitivity and reproducibility, particularly for the discrimination between HER2-low score 1+ and “HER2-zero” (i.e., IHC score 0), which comprises also the subset of “HER2 ultra-low” (i.e., score 0 with incomplete and faint staining in ≤10% of tumor cells) (Sajjadi et al., 2022).