(A) In vitro viability assay for XTT. Percentages of viability (means ± standard errors of the means [SEM] as measured at 492 nm) relative to the viability of the control are plotted on the y axis, and increasing concentrations of caspofungin (0 to 1,024 μg/ml) are plotted on the x axis. A decrease in the paradoxical attenuation of caspofungin activity was observed in the Δmkc1/Δmkc1 homozygous-knockout mutant compared to that of C. albicans strain ATCC 90028 and the parent strain CAI4 (*, P < 0.001 for Δmkc1/Δmkc1 at concentrations of 8 to 32 μg/ml). ▪, C. albicans strain ATCC 90028; •, CAI4; ○, Δmkc1/Δmkc1 strain. (B) Relative MKC1 gene expression in Candida albicans strain ATCC 90028 (mean + SEM). Changes in gene expression compared to that of cells not exposed to caspofungin are plotted on the y axis and capsofungin concentrations on the x axis. Expression levels were normalized to 18S rRNA.