is a methylotrophic yeast capable of metabolizing methanol as its sole carbon source. Growth in methanol-containing medium results in dramatic induction of genes in the alcohol oxidation pathway including alcohol oxidase (AOX), formaldehyde dehydrogenase (FLD), and dihydroxyacetone synthase (DHAS). These proteins may comprise up to 30% of the biomass. Investigators have exploited these methanol-dependent genes to generate tightly regulated expression vectors. Most vectors use the strong and tightly regulated promoter to drive heterologous protein expression. Obtaining integrated transformants requires more DNA than transformations into , where the gene is expressed from episomal plasmids; however, transformants are extremely stable and can be stored for many years.