56 estrogen receptor positive breast cancer tissues from patients treated with tamoxifen upon recurrence (32 Good outcome vs 24 Poor outcome) have been microdissected to obtain a pure population of epithelial tumor cells. Breast tumor enriched tissues have been sonicated and proteins have been in-solution digested and analyzed via a LTQ-Orbitrap coupled online with a nano-LC system. Thermo output .RAW files were analyzed through MaxQuant (version 1.4.1.2) using the human proteome FASTA (version 2012-09, human canonical proteome) as search library.