Abstract: Improved methods of treatment are provided for patients suffering from cancer. The methods identify whether a tumor will be responsive to treatment with a therapeutic regime that includes anti-Her2 therapeutic agents. A specific Her2 fragment peptide is precisely quantitated by SRM-mass spectrometry directly in tumor cells collected from tumor tissue that was obtained from a cancer patient and compared to a reference level in order to determine if the cancer patient will positively respond to treatment with a therapeutic agent that specifically targets the Her2 protein.