The human growth hormone (GH) is a pleiotropic protein that acts in different tissues such as: brain, spinal roots and peripheral nerves, heart, blood vessels, ovary, testis, intestine, kidney, muscle, bone, thymus, and liver. The human GH/CS locus is located in chromosome 17 and includes two genes for GH (GH-1 and GH2) and three chorionic somatomammotropin, hCS-1, hCS-2, hCS-5, currently called placental lactogens. GH-1 (GH-N) is expressed in pituitary and GH-2 (CG-V) in placenta. Human GH-1 is synthetized by the somatotrophs in the anterior pituitary and its release is regulated by the growth hormone releasing hormone (GHRH) and GHRH receptor (GHRHR). The GHRH is produced by the hypothalamus and the binding to the GHRH receptor releases the GH-1 stored in secretory granules. GH-1 is negatively regulated by GH release-inhibiting factor (GHRIF), and somatostatin (SST). The human growth hormone gene 1 is composed of five exons and four introns. The mature transcript encodes a biologically active form of 22 kD. Aberrant splicing in exon three produces a 20 kD active protein, and elimination of exon three results in a 17.5 kD protein that inhibits the secretion of the full active form. GH-1 promotes thymus development and engraftment of human and murine T cells in SCID mice; also induces T and B cell proliferation. GH-1 acts directly in some cells such as osteoblasts, epiphyseal growth-plate chondrocytes; some effects of GH-1 are indirectly induced by IGF-1 produced in the liver in response to GH-1. Thus, GH-1 and IGF-1 have a synergistic effect with GM-CSF in human granulopoiesis in vitro. GH-1 binds to the growth hormone receptor (GHR) and this binding induces dimerization of GHR to form a dimeric GHR:GH complex. The dimerization induces interaction with Janus2 and subsequently signals activation.