STAT proteins ( called Signal Transducers and Activators of Transcription ) are a group of proteins involved in the transmission of information from the cell surface to the nucleus and activate gene transcription . They are activated in response to a number of cytokines , growth factors and hormones. Until now identified six STAT proteins : STAT1 , 2, 3 , 4, 5a/5b , 6 So far it was found that they play an important role in many physiopathological processes , starting from the immune responses, cell cycle regulation and malignant transformation of cells. There are also reports that alterations in signaling through the JAK- STAT in some diseases of the hematopoietic system . The paper discusses the structure, functioning and methods of inactivation of STAT proteins . Moreover presents the latest information on the role of STAT5 proteins in the functioning of the hematopoietic system , immune , and his participation in the process of carcinogenesis .