Correlation between HER2 gene copy number and immunohistochemistry categories in HER2-negative breast cancer: diagnostic utility for differentiating HER2-null, ultralow, and low tumors

HER2 gene copy number was positively correlated with protein expression as reflected by IHC categories. Although HER2 gene copy number was statistically higher in HER2-low than in HER2-null tumors, the substantial overlap in copy number ranges likely limits its utility in distinguishing HER2-low from HER2- null BCs.