Signal transducer and activator of transcription 5(STAT5)was an important signal transducer and activator of transcription, and could be activated by aplethora of cytokines and polypeptide hormones resulting in transcriptional activation of specific target genes.It participated in the regulation of proliferation,differentiation and apoptosis of cell and played an important role in growth,hematopoiesis and immunological response.Furthermore,the activity of STAT5 was related to occurrence and development of tumor.This review summarized the structure and biological functions of STAT5.