The ESP® yeast protein expression and purification system uses the yeast Schizosaccharomyces pombe as the expression host and the glutathione S-transferase (GST) peptide as the protein purification tag. This system provides an easy alternative to protein production in E. coli. Proteins expressed in E. coli may lack proper biological function and antigenicity because of the absence of eukaryotic posttranslational modifications. S. pombe is a single-cell eukaryotic organism with properties similar to higher eukaryotic organisms. These properties, such as chromosome structure and function, cell-cycle control, RNA splicing and codon usage, make S. pombe ideal for the production of eukaryotic proteins. Also, eukaryotic proteins expressed in S. pombe are more likely to be folded properly, which improves the specific activity and can eliminate protein insolubility problems found in E. coli expression systems.