In the pathogenic yeast Candida albicans, the zinc cluster transcription factor 
Upc2p has been shown to regulate the expression of ERG11 and other genes 
involved in ergosterol biosynthesis upon exposure to azole antifungals. ERG11 
encodes lanosterol demethylase, the target enzyme of this antifungal class. 
Overexpression of UPC2 reduces azole susceptibility, whereas its disruption 
results in hypersusceptibility to azoles and reduced accumulation of exogenous 
sterols. Overexpression of ERG11 leads to the increased production of lanosterol 
demethylase, which contributes to azole resistance in clinical isolates of C. 
albicans, but the mechanism for this has yet to be determined. Using genome-wide 
gene expression profiling, we found UPC2 and other genes involved in ergosterol 
biosynthesis to be coordinately upregulated with ERG11 in a 
fluconazole-resistant clinical isolate compared with a matched susceptible 
isolate from the same patient. Sequence analysis of the UPC2 alleles of these 
isolates revealed that the resistant isolate contained a single-nucleotide 
substitution in one UPC2 allele that resulted in a G648D exchange in the encoded 
protein. Introduction of the mutated allele into a drug-susceptible strain 
resulted in constitutive upregulation of ERG11 and increased resistance to 
fluconazole. By comparing the gene expression profiles of the 
fluconazole-resistant isolate and of strains carrying wild-type and mutated UPC2 
alleles, we identified target genes that are controlled by Upc2p. Here we show 
for the first time that a gain-of-function mutation in UPC2 leads to the 
increased expression of ERG11 and imparts resistance to fluconazole in clinical 
isolates of C. albicans.