Protropin (somatrem for injection) is a polypeptide hormone produced by recombinant DNA technology. Protropin (somatrem) has 192 amino acid residues and a molecular weight of about 22,000 daltons. The product contains the identical sequence of 191 amino acids constituting pituitary-derived human growth hormone plus an additional amino acid, methionine, on the N-terminus of the molecule. Protropin (somatrem) is synthesized in a special laboratory strain of E. coli bacteria which has been modified by the addition of the gene for human growth hormone production.