Differences in methodology, disease stage (early vs. advanced), tumor specimen (resection specimen vs. biopsy), site of tumor specimen (primary vs. metastases), IHC scoring system (qualitative vs. semiquantitative), threshold definition of IHC overexpression (provided vs. not), and/or choice of tumors in which the ISH test was applied (HER2 overexpression vs. no overexpression) may explain the wide range of both HER2 expression (0 to 82 %) and amplification (0 to 100 %) positivity reported in this review. This evidence concerns the gene ERBB2 and neoplasm.