If changes in gene expression have played an important role in evolution, changes in gene expression should be abundant within and between species. Consistent with this idea, extensive levels of genetic variation in gene expression have been found in all organisms examined, from yeast to humans. Yet, the functional significance of this variation remains largely unknown. The simplest explanation is that the variation exists because the majority of slight changes in gene expression levels have little if any effect on fitness. We are using two approaches to understand the significance of population genetic variation in gene expression. First, we are working towards identifying the genetic basis of expression variation in order to characterize the mechanism by which expression variation arises as well as any downstream phenotypic consequences. Second, we are mapping the genetic basis of a variety of drug-dependent growth phenotypes in order to determine whether any direct or indirect changes in gene expression are necessary for the production of variation in drug-sensitivity.