To study the effect of carbon source on gene expression levels, P. stipitis has been cultivated aerobically in well controlled bioreactors using either glucose or xylose as a carbon source.  The effect of oxygen level has on gene expression has also been examined. Unlike S. cerevisiae, which regulates fermentation by sensing the presence of fermentable sugars, P. stipitis induces fermentation in response to oxygen limitation. P. stipitis has been cultivated on both glucose and xylose under oxygen limited conditions. Comparison of gene chip results from these fermentations with the aerobic cultivations will allow for the identification of genes that are induced under oxygen limitation and those that may be affected both by oxygen level and carbon source.  These studies will provide invaluable insight into growth on xylose by P. stipitis and the genes responsible for fermentation of this extremely abundant sugar. Identification of these genes will yield targets for the future engineering of improved xylose fermenting yeasts.