P. stipitis CBS 6054 is unusual in that it is the best of only a few yeasts known to ferment xylose to ethanol in high yield. It is related to several yeasts that are found as endosymbionts of beetles that inhabit and degrade white-rotted wood. Unlike Saccharomyces cerevisiae, which regulates fermentation by sensing the presence of fermentable sugars such as glucose, Pichia stipitis induces fermentative activity in response to oxygen limitation.