There are seven mammalian STAT family members that have been identified: STAT1, STAT2, STAT3, STAT4, STAT5 (STAT5A and STAT5B), and STAT6.
Signal Transducer and Activator of Transcription 5 refers to two highly related proteins, STAT5A and STAT5B, which are part of the seven-membered STAT family of proteins.