ERBB2 and neoplasm: However, because the cellularity of HER2 amplification is quite homogeneous, with all positive cases displaying amplification in >50% of cells, a 3+ HER2 score in more than 50% of tumor cells by IHC or a 2+ HER2 score and a HER2/CEP17 ratio >2 in more than 50% of tumor cells by FISH are required to be considered a HER2-positive diagnosis.