 The enzyme arginine deiminase ADI metabolizes arginine into citrulline Cunin 1986. However, ADI is only found in microbes and not in humans. ADI is therefore, highly immunogenic and has a short serum half-life following injection. These potential drawbacks microbial source and thus viewed as foreign by the human immune system, and a short serum half-life can be overcome by covalent attachment of polyethylene glycol PEG to argininedeiminase and termed this drug ADI-PEG 20. 