Signal transducer and activator of transcription 5A (STAT5A) and Signal transducer and activator of transcription 5B (STAT5B) dimers bind to similar core gamma-interferon activated sequence (GAS) motifs (Soldaini et al., 2000). STAT5A/B also form homo- and hetero-tetramers with distinct or expanded DNA-binding properties. Genes that are regulated by STAT5 include Interleukin-2 receptor alpha (IL2RA) (John et al. 1996), TNFSF11 (RANKL), Connexin-26 (GJB2) and Cyclin D1 (Hennighausen & Robinson, 2005). A comprehensive listing of hepatic STAT5B regulated genes is available from microarray/STAT5B knockout mice (Clodfelter et al. 2006), and similarly for STAT5-dependent genes regulated by the GH receptor (Rowland et al. 2005, Barclay et al. 2011).