Complex Human Proteins by Using Molecular Biotechnology Techniques - Most of the proteins are simple and can be expressed in bacteria and yeast. But proteins of medical interest are considerably more complex in structure, and it is difficult to produce biologically active proteins in bacteria and yeast.

Mammalian cells can be useful but they are ficky and expensive to grow. Thus much of the effort has been devoted to setting up fermentor systems for large scale culture of mammalian cells.

The first drug to be produced commercially by mammalian cell culture was tissue plasminogen activator or tPA, which is administered to heart attack victims. Tissue plasminogen activator is a protease, an enzyme that cleaves other proteins.