STAT5(Signal transducer and activator of transcription 5) also known as STAT5A, MGF, is a protein that serve the dual function of signal transducers and activators of transcription in cells exposed to signaling polypeptides. Hou et al. (1995) cloned the human STAT5 cDNA from an umbilical vein endothelial cell library and found that it encodes a 794-amino acid polypeptide with a predicted mass of approximately 90.5 kD. To analyze the possible role of STAT5 in human GH-induced proliferation, Friedrichsen et al. (2001) expressed a dominant-negative STAT5 mutant, STAT5A-delta-749, in INS-1 cells under the control of a doxycycline-inducible promoter by stable transfection. Chromatin immunoprecipitation analysis confirmed direct binding of STAT5 to the HIF2A promoter 344 bp upstream of the HIF2A start site. STAT5 overexpression independently resulted in erythroid commitment in megakaryocytic-erythroid progenitors, which was abrogated by knockdown of GATA1. STAT5 is responsible for the immediate-early induction of the long isoform of the BCL2-related protein in erythroid cells through direct binding to the promoter of the BCLX gene.