S. cerevisiae cannot utilize pentose sugars, so S. cerevisiae
has been genetically engineered over the past several decades
to allow pentose sugar fermentation. The introduction of heterologous genes encoding xylose
reductase and xylitol dehydrogenase from Pichia stipitis, and xylulokinase
from S. cerevisiae has provided strains of S. cerevisiae that
can ferment xylose