Yeast systems have been a staple for producing large amounts of proteins for industrial and biopharmaceutical use for many years. Yeast can be grown to very high cell mass densities in well-defined medium. Recombinant proteins in yeast can be over-expressed so the product is secreted from the cell and available for recovery in the fermentation solution. Proteins secreted by yeasts are heavily glycosylated at consensus glycosylation sites. Thus, expression of recombinant proteins in yeast systems historically has been confined to proteins where post-translations glycosylation patterns do not affect the function of proteins. Several yeast expression systems are used for recombinant protein expression, including Sacharomyces, Scizosacchromyces pombe, Pichia pastoris and Hansanuela polymorpha.