STAT5A and breast cancer: It was reported that signal transducer and activator of transcription 5 (STAT5), Src and EGFR play important roles in estrogen-stimulated proliferation of ER-positive breast cancer cells [30]; estrogen-induced Src activation and Src-dependent phosphorylation of EGFR-Tyr-845 recruit STAT5 as a downstream effector of phosphorylated EGFR-Tyr-845 [30].