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A negative control is a comparison expected to lack the response attributed to the tested cause. In a generic analytical measurement, a blank without the target substance reveals background signal or contamination introduced by reagents and handling. A vehicle control can isolate effects of the carrier used to deliver an intervention. A clean negative control supports interpretation but cannot prove that every unwanted influence is absent; the control must share the relevant conditions with the test samples.^[ref]

ToxB greatly reduces recognizable actin cables and actin patches. Untreated biological cells contain concentrated cortical structures, especially in buds, whereas treated biological cells show only weak diffuse staining even at ten times the exposure. The disappearance concerns actin filament organization and detectable F-actin, not necessarily destruction of the actin protein itself. actin monomer sequestration could leave substantial total actin present while eliminating most visible polymer.

Loss of actin cables should impair directed secretory-vesicle delivery and polarized cell growth; loss of actin patches should impair actin-dependent endocytosis. Disruption of the actomyosin machinery can also compromise cytokinesis. The rounded, poorly polarized appearance is consistent with these effects, but the images do not independently measure secretion, division rate or viability. A vehicle control and equal-exposure images are needed for quantitative fluorescence comparison; a longer exposure is useful for finding residual structures but prevents direct comparison of the displayed brightnesses.