ERBB2 and breast cancer: However, as a semi-quantitative test, HER2 IHC testing was primarily developed to help separate high levels of HER2 expression [~2 millions of molecules per cell, corresponding to HER2 IHC score 3+] from lower level HER2 expression (~20,000–500,000 molecules per cell, corresponding to HER2 IHC 0–2+) and may not be an ideal method for detecting HER2-low BC.