In the adjuvant setting expression of YB-1 is indicative of reduced BCSS. YB-1 expression was evaluated in a nested cohort of 671 tamoxifen-treated patients. Twenty-eight percent of patients expressed YB-1 (185/671), in whom its expression was associated with reduced BCSS. Survival was assessed over 5 years, because this is the standard duration of tamoxifen administration. ER, estrogen receptor; YB-1, Y-box binding protein-1.