We selected a total of 47 breast cancer tissues which displayed at least (≥) 50% tumor area from patients that manifested either good or poor outcome to tamoxifen treatment, which was defined as whether patients manifested disease progression before (≤; poor) or after (>; good) 6 months. This subset comprised 28 and 19 patients who manifested good and poor outcome, respectively (Table 1). An independent set of 26 breast cancer patient derived sera was selected as verification cohort. Sera were collected both prior start of first line tamoxifen therapy (Supplemental Table 1), and comprised 14 good and 12 poor outcome patients. Both cohorts were analyzed in singlicate as biological replicates. In order to assess the efficacy of immuno-capture coupled to MRM MS, a set of 8 ER positive tumor specimens (≥ 50% tumor area) was analyzed in two triplicated series (i.e. 8 x 3 replicates by standard MRM, 8 x 3 replicates by iMRM).