The yeast Saccharomyces cerevisiae can be grown in the laboratory with ease and at relatively low expense, and can be propagated in large scale fermentation cultures when preparation of larger amounts of a recombinant protein is desired. The S. cerevisiae genome can be manipulated to reduce proteolysis or prevent unwanted posttranslational processing of recombinant proteins (1 ,2 ). Genes that control protein degradation and glycosylation have been identified, and mutants are available for constructing genetic backgrounds appropriate for overcoming problems specific to expression of the recombinant protein of choice (3 –5 ). The availability of such mutations combined with the ability to introduce and express foreign genes, isolate temperature sensitive mutants, or alter nonessential genes by targeted disruption makes the yeast Saccharomyces cerevisiae an attractive host for the production of heterologous recombinant proteins.