STAT5A and breast cancer: We first performed fluorescence immunohistochemistry for nuclear-localized and tyrosine-phosphorylated Stat5a/b (Nuc-pYStat5) on 323 breast cancer specimens represented in tissue microarray format on a single histological slide (Cohort 1) that was coimmunostained for pYStat5 and pancytokeratin, and counterstained with DAPI for nuclear detection.