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Single cell eukaryotic yeast has complete gene expression regulation mechanism and the ability to process and modify expression products.Saccharomyces.Cerevisiae was first recognized in molecular genetics and was the first yeast host to be expressed as an exogenous gene.Saccharomyces cerevisiae in 1981 expressed the first foreign genes, interferon gene, then there is a series of exogenous gene expression in the system although interferon and insulin have been using saccharomyces cerevisiae and is widely used in mass production when using saccharomyces cerevisiae preparation, laboratory results are very encouraging, but expanded from laboratory to industrial scale, its production decline rapidly.The reason was that the selection pressure of high copy number of vettel plasmids disappeared and plasmids became unstable and copy number decreased.Copy number is a necessary factor for efficient expression, so the decrease of copy number also directly leads to the decrease of exogenous gene expression.At the same time, laboratory media are complex and expensive, resulting in a decline in yield when industrial-scale acceptable media are used.In order to overcome the limitation of saccharomyces cerevisiae, in 1983 Wegner et al. first developed the second generation yeast expression system represented by methylotrophic yeast (methylotrophic).Methylotrophic yeasts include: Pichia, Candida, etc.The exogenous gene expression system of Pichia pastoris (Pichia pastoris) as host has been developed most rapidly and widely used in recent years.Pichia pastoris system is widely used because it has the characteristics of common yeast.