Human growth hormone (hGH, somatotropin), a protein hormone from the anterior lobe of the pituitary gland, comprises a single polypeptide chain of 191 amino acid residues. The growth hormone amino acid sequence varies considerably between species, and non-primate growth hormones have little activity in man. Its main function is the stimulation of somatic and bone growth, as well as an increase in the size and mass of organs and tissues.