These authors performed an isobaric Tandem Mass Tag (TMT) label-based quantitative proteomic analysis of six serum samples from primary HER2-positive breast cancer cases, including three trastuzumab-based therapy-resistant and three trastuzumab-based therapy responsive cases, to discover the serum fingerprint proteins that are related to the trastuzumab-based therapy response in a parallel manner. Here, ERBB2 is linked to breast cancer.