ERBB2 and neoplasm: Finally, some of the discordances in the HER2 status between the primary and recurrent tumours we observed in our study may be due to inter-observer variability, i.e., variations in the interpretation of HER2 testing results, especially when it comes to differentiating between HER2 IHC 0 and 1+ scores, which can be very challenging [20, 26, 39].