Fluorescence immunohistochemistry facilitated identification of patients with extremely low Nuc-pYStat5 who were at markedly elevated risk of breast cancer recurrence, whereas pathologist-evaluated chromogen immunohistochemistry did not readily have sufficient sensitivity to distinguish the lowest Nuc-pYStat5-expressing sub-population of tumors and failed to detect an association with clinical outcome. Here, NUCB1 is linked to breast carcinoma.