Updated publication reference for PubMed record(s): 26958999, 27030549. A total of 38 flash-frozen breast cancer tissues were collected from three medical centers in the Netherlands (i.e. National Cancer institute: n=19; Radboud University Medical Center: n=4; Erasmus University Medical Center: n=15). All tissues were analyzed as laser capture microdissected (LCM) and whole samples (whole tissue lysate: WTL). All gathered samples were primary estrogen receptor (ER) tumors, which were subdivided in two patient groups according to outcome to first line (i.e. recurrent disease setting) tamoxifen therapy based on time-to-progression (TTP): patients which manifested progression of disease before (i.e. TTP <=) 6 months were defined as "poor outcome", while patients which manifested progression after (i.e. TTP>) 6 months were defined as "good outcome". All patients did not receive adjuvant hormonal therapy (i.e. tamoxifen or aromatase inhibitors post-surgical resection of primary tumor). The LCM subset derives from PXD000484 (Erasmus Medical Center samples) and PXD000485 (National Cancer Institute and Radboud University Medical Center).