ERBB2 and neoplasm: The findings in the literature are biased by a series of inherent limitations that affect the possibility to draw consistent results—mostly (1) the retrospective fashion of available data, (2) intra- and inter-tumor heterogeneity, and (3) discrepancies among assessment methods used in different studies, namely antibody clones, evaluation criteria, and cut-offs adopted for the definition of HER2 positivity [37,45,62,63].