(19) These methods of multiple reaction monitoring mass-spectrometry, which evaluate formalin-fixed-paraffin-embedded breast cancer tissues, have been found to both offer numerical quantification of HER2 levels that correlates well with established IHC classification, even at low HER2 expression levels (20, 21), as well as possibly distinguish between HER2-positive and HER2-negative samples with high specificity and sensitivity (22). This evidence concerns the gene ERBB2 and breast cancer.