Low levels of nuclear Stat5a (Nuc-Stat5a) predict poor response to antiestrogen therapy. (A, B) Nuc-Stat5a was detected by DAB-chromogen IHC and pathologist scoring of whole-tissue sections from node-negative breast cancer patients treated with antiestrogen monotherapy (Material V). The lack of nuclear localization of Stat5a predicted (A) poor breast cancer-specific survival (CSS) and (B) reduced time-to-recurrence (TTR) of breast cancer. (C, D) Nuc-Stat5a expression levels were measured with immunofluorescence and quantified with AQUA in node-negative and -positive breast cancer patients treated with antiestrogen monotherapy (Material VI). Low levels of Nuc-Stat5a were predictive of (C) poor cancer-specific survival (CSS) and (D) reduced TTR of breast cancer. Kaplan-Meier plots with censored cases (+) and number of patients per group indicated.