Viability assays are designed to measure activities attributable to cellular maintenance and survival. These activities are typically metabolic biomarkers such as ATP and mitochondrial reductase potential, but can include homeostatic "housekeeping" enzyme activities. The underlying assay premise is that these activities are directly proportional to viable cell number after a treatment period. During relatively short incubation periods (8 hr or less), a reduction or complete cessation of these biomarker activities is strongly indicative of overt cytotoxicity by catastrophic membrane damage (i.e. primary necrosis) [11] . A reduction in biomarker activity compared to control in longer incubations indicates either a reduction in normal cellular division rate (cell-cycle arrest) or cell death by programmed elimination mechanisms such as apoptosis [12, 13] . Because viability assays measure the relative number of cells remaining after treatment, they offer significant utility even during long compound-contact periods (72 hr).