STAT5A and neoplasm: STAT (signal transducer and activator of transcription) transcription factor regulates cell growth, differentiation, and survival. There are 7 mammalian family members: STAT1, STAT2, STAT3, STAT4, STAT5 (STAT5A and STAT5B), and STAT6. Dysregulation of these proteins leads to increased angiogenesis, enhanced survival of tumors, and immunosuppression. STAT proteins are also involved in the development and function of the immune system and play a role in maintaining immune tolerance and tumor surveillance.