Life sciences firm Sigma-Aldrich has a line of knockout rats produced with its line of zinc-finger nucleases, artificial enzymes that can knock out certain sequences of a genome. The new rat models a a term scientists use to describe an animal being used for medical research a can be used to study various neurobiological disorders, like Parkinson's disease, schizophrenia, dementia and depression. Rat physiology is very similar to humans. During the past year, the company worked with the Michael J. Fox Foundation to make the first Parkinson's rats. Genes known to be associated with Parkinson's are knocked out, providing a rat with the biochemical, physiological and behavioral characteristics of Parkinson's. Sigma went with rats because mice don't have very complex nervous systems, according to Dave Smoller, president of Sigma's Research Biotech division. aParkinson's models in mice don't shake, so maybe rats will be a better model,a he said. Sigma also sells off-the-shelf rats, and offers rats and mice with knockout genes made to order , Smoller said. aIf someone has enough they want, we will make it for them,a he said. The Parkinson's rats are not available yet, but Sigma's knockout rat catalog has about a dozen other specimens. The Mdr1a knockout rat , for instance, can be used to study cancer. The animals are shipped worldwide in climate-controlled containers to aensure safe, clean and comfortably stress-free travel,a according to Sigma's website.