As unicellular eukaryotes, yeasts combine the benefits of the bacterial system with eukaryotic characteristics of gene expression, but posttranslational modifications are often performed incorrectly, for example, resulting in misfolded and hyperglycosylated products. Mammalian cell cultures for the expression of recombinant proteins exhibit a number of advantages over the microbial systems; however, animal cell cultures are often difficult to handle, particularly in large-scale production, and costs for media and cultivation are considerably high, leading to a price range of $150–1000 per gram protein before purification. A major disadvantage of mammalian cell cultures is the risk of contamination of the isolated protein with potentially pathogenic agents such as viruses and prions. Methods using transgenic animals delivering recombinant proteins in their milk have gained much attention (Maga, 2005). But the establishment of transgenic animals is still extremely time-consuming and very expensive.