Methods of treating breast cancer are provided where a quantitative Her2 assay is used to identify whether a breast tumor will be responsive to treatment with anti-Her2 therapeutic agents such as lapatinib and trastuzumab, followed by selection of a suitable treatment regimen and administration of the regimen. A specific Her2 fragment peptide is precisely quantitated by SRM-mass spectrometry directly in breast tumor cells collected from breast tumor tissue that was obtained from a cancer patient and compared to a reference level in order to determine if the breast cancer patient will positively respond to treatment with a therapeutic agent that specifically targets the Her2 protein.