The Signal transducer and activator of transcription 5A(STAT5A) and Signal transducer and activator of transcription 5B(STAT5B) forms are encoded by 2 closely-related genes. They are thought to be present largely as monomers in unstimulated cells but rapidly form homo- and hetero-dimers upon stimulation (Cella et al. 1998). Tyrosine phosphorylation of STAT monomers allows dimers to form through reciprocal phosphotyrosine-SH2 interactions. The dimers translocate to the nucleus and bind to STAT-specific DNA-response elements of target genes to induce gene transcription (Baker et al.2007). STAT5A/B homo- and hetero-tetramers have also been shown to occur downstream of Interleukin-2 (IL2) and may have a distinct or expanded target repertoire from STAT5A/B dimers. Although STAT5 andA STAT5B are highly homologous at the DNA and protein levels, each has unique functions, as demonstrated by studies comparing mice lacking one isoform or the other. However, it is also known that STAT5A and STAT5B share a number of functions and that the phenotype of mice lacking both STAT5A and STAT5B is more severe than those lacking either one individually, which suggest that there may be some redundancy or that they cooperate in order to achieve the full spectrum of STAT5-dependent activities (Moriggl et al. 1999, Teglund et al. 1998).