STAT5 alpha is a member of the STAT family of transcription factors. In response to cytokines and growth factors, STAT family members are phosphorylated by the receptor associated kinases, and then form homo- or heterodimers that translocate to the cell nucleus where they act as transcription activators. STAT5 alpha is activated by, and mediates the responses of many cell ligands, such as IL2, IL3, IL7 GM-CSF, erythropoietin, thrombopoietin, and different growth hormones. Activation of STAT5 alpha in myeloma and lymphoma associated with a TEL/JAK2 gene fusion is independent of cell stimulus and has been shown to be essential for the tumorigenesis. The mouse counterpart of STAT5 alpha is found to induce the expression of BCL2L1/BCL-X(L), which suggests the antiapoptotic function of this protein in cells. STAT5 alpha, along with STAT5 beta, were identified in mouse. The amino acid sequences of STAT5 alpha and STAT5 beta show 96% sequence similarity, and both proteins are co expressed in most tissues in virgin and lactating mice. However, differential accumulation of STAT5 alpha and STAT5 beta mRNA has been reported for both muscle and mammary tissue. STAT5 alpha is critically involved in a variety of physiological functions, including reproduction, lactation, immune function and somatic growth.