S important to understand mechanisms of GH action in order to devise strategies to enhance its positive physiological effects while limiting its negative impact on human disease. Like other members of the cytokine receptor family, upon ligand binding the GH receptor engages and stimulates the Jak Stat signaling pathway [7,9?1]. GH binding induces the receptor-associated tyrosine kinase, Jak2 [7,9] to phosphorylate tyrosine residues on the intracellular part of the receptor [1,8,12], leading to the recruitment of several Stats, as well as other signaling molecules [1,8,12]. Stats comprise a group of seven related proteins in mammals [7,9?1], with the first members being characterized as signaling agents for interferons a/b and c [13,14]. Subsequent studies have broadened the biological importance of this protein family ascritical components of multiple physiological and patho-physiological Felypressin processes [7,9?1]. Stats are typically found in the cytoplasm of responsive cells prior to hormone or cytokine stimulation. After being recruited to phosphorylated tyrosine residues on intracellular segments of activated receptors, they become phosphorylated on a tyrosine near the Stat COOHterminus by a receptor-linked tyrosine protein kinase, usually Jak13, or Tyk2 [7,9,10]. After dissociation from the receptor docking site, Stats form dimers via reciprocal interactions of the Src homology 2 domain on one Stat molecule with the phosphorylated tyrosine on the other [9], and are translocated into the nucleus, where they bind as dimers to specific DNA sites in chromatin [7,9?11]. Stats recognize the palindromic DNA sequence, 59TTCNxGAA-39 (where N is any deoxynucleotide, and x = 2?), but with distinct preferences depending on the individual Stat [9,15]. Despite clear evidence that multiple signaling pathways act downstream of the GH receptor, recently identified inactivating molecular lesions in the STAT5B gene in humans with impaired growth [16,17], targeted gene knockouts of the GH receptor [18,19] and Stat5b in mice [20?2], and biochemical and molecular studies [23], have collectively implicated Stat5b as the essential signaling intermediate responsible for many of the critical biological actionsDefining GH-Activated Stat5b Enhancersof GH. For example, a key agent of GH-regulated somatic growth and tissue repair is 23727046 insulin-like growth factor-I (IGF-I), a highly conserved 70-amino acid secreted protein [2,24], whose gene transcription is rapidly and potently induced by GH via Stat5b [25,26]. However, unlike most other genes whose transcription is acutely activated by GH through Stat5b, such as Socs2, Cish, and Igfals in 
rodents, in which functionally critical Stat5b binding sites are located within the proximal promoters, there are no Stat5b Methionine enkephalin web transcriptional response elements within either of the two promoters of the Igf1 gene [27,28]. Rather, several distinct GHinducible Stat5b binding domains have been mapped to introns and to distal regions of human IGF-I and rat and mouse Igf1 loci [29?4]. Although some of these elements appear to possess chromatin characteristics of transcriptional enhancers [34], their biochemical properties have not been elucidated to date. Here we have evaluated the biochemical and functional characteristics of the multiple dispersed chromosomal Stat5b binding domains in the rat Igf1 locus, as a means to understand how they contribute to control of IGF-I gene transcription by GH. We find that each Stat5b element has distinct tra.S important to understand mechanisms of GH action in order to devise strategies to enhance its positive physiological effects while limiting its negative impact on human disease. Like other members of the cytokine receptor family, upon ligand binding the GH receptor engages and stimulates the Jak Stat signaling pathway [7,9?1]. GH binding induces the receptor-associated tyrosine kinase, Jak2 [7,9] to phosphorylate tyrosine residues on the intracellular part of the receptor [1,8,12], leading to the recruitment of several Stats, as well as other signaling molecules [1,8,12]. Stats comprise a group of seven related proteins in mammals [7,9?1], with the first members being characterized as signaling agents for interferons a/b and c [13,14]. Subsequent studies have broadened the biological importance of this protein family ascritical components of multiple physiological and patho-physiological processes [7,9?1]. Stats are typically found in the cytoplasm of responsive cells prior to hormone or cytokine stimulation. After being recruited to phosphorylated tyrosine residues on intracellular segments of activated receptors, they become phosphorylated on a tyrosine near the Stat COOHterminus by a receptor-linked tyrosine protein kinase, usually Jak13, or Tyk2 [7,9,10]. After dissociation from the receptor docking site, Stats form 
dimers via reciprocal interactions of the Src homology 2 domain on one Stat molecule with the phosphorylated tyrosine on the other [9], and are translocated into the nucleus, where they bind as dimers to specific DNA sites in chromatin [7,9?11]. Stats recognize the palindromic DNA sequence, 59TTCNxGAA-39 (where N is any deoxynucleotide, and x = 2?), but with distinct preferences depending on the individual Stat [9,15]. Despite clear evidence that multiple signaling pathways act downstream of the GH receptor, recently identified inactivating molecular lesions in the STAT5B gene in humans with impaired growth [16,17], targeted gene knockouts of the GH receptor [18,19] and Stat5b in mice [20?2], and biochemical and molecular studies [23], have collectively implicated Stat5b as the essential signaling intermediate responsible for many of the critical biological actionsDefining GH-Activated Stat5b Enhancersof GH. For example, a key agent of GH-regulated somatic growth and tissue repair is 23727046 insulin-like growth factor-I (IGF-I), a highly conserved 70-amino acid secreted protein [2,24], whose gene transcription is rapidly and potently induced by GH via Stat5b [25,26]. However, unlike most other genes whose transcription is acutely activated by GH through Stat5b, such as Socs2, Cish, and Igfals in rodents, in which functionally critical Stat5b binding sites are located within the proximal promoters, there are no Stat5b transcriptional response elements within either of the two promoters of the Igf1 gene [27,28]. Rather, several distinct GHinducible Stat5b binding domains have been mapped to introns and to distal regions of human IGF-I and rat and mouse Igf1 loci [29?4]. Although some of these elements appear to possess chromatin characteristics of transcriptional enhancers [34], their biochemical properties have not been elucidated to date. Here we have evaluated the biochemical and functional characteristics of the multiple dispersed chromosomal Stat5b binding domains in the rat Igf1 locus, as a means to understand how they contribute to control of IGF-I gene transcription by GH. We find that each Stat5b element has distinct tra.