Assessing the level of HER2 expression in breast tissue, i.e., HER2 status, is routinely practiced based on the HER2 IHC staining of the formalin-fixed, paraffin-embedded (FFPE) tissue sections and helps predict the prognosis of breast cancer and its response to HER2-directed immunotherapies [5, 30–34]. This evidence concerns the gene ERBB2 and breast cancer.