Protein Production requires the gene for the desired human protein to be inserted into an appropriate host cell. The most commonly used cells are the bacterium Escherichia coli (E. coli) and the ovary cells from the Chinese hamster. The process is highly challenging and specialized, as living cells are very sensitive: environmental factors play a big role in the yield, the quality or even the structure of the active agent produced. This is why biotechnological products produced by a different manufacturer can never be identical but only “similar” to an existing protein drug. Genetically modified cells are reproduced and stored in a master cell bank at very low temperatures. When needed, cells are transferred from the deep-frozen cell bank to increasingly large steel containers containing liquid nutrient medium and then cultured. The culture media contain many different components that are optimized for the manufacture of the desired protein. Purification, quality control and formulation of the sensitive protein products are complex and specialized steps in themselves – 99.9 percent purity of the therapeutic protein is required for marketing authorization.