Depending upon the goals of the screen, either viability or true cytotoxicity assay chemistries can be employed. Cytotoxicity assays based on membrane integrity changes are positive-readout assays most typically indicated for shorterterm exposure models (48 hr or less). These assays may not always accurately estimate the absolute degree of early or late stage cytotoxicity due to the kinetics of biomarker emergence or degradation. Viability assays measure the level of biomarker activity inversely correlated with cytotoxicity and therefore may be used at any endpoint during a compound/cell incubation period.