How It Works Molecular Probes® LIVE/DEAD® Fixable Dead Cell Stain Kits are based on the reaction of a fluorescent reactive dye with cellular proteins (amines). Viable Cells — Because dyes can not penetrate the cell membrane, only cell surface proteins are available to reactive with dye resulting in dim staining (Figure 1). Dead Cells — The reactive dye can permeate damaged membranes and stain both the interior and exterior amines resulting more intense staining (Figure 1). The difference in fluorescence intensity between the live- and dead-cell populations is typically greater than 50-fold, thereby allowing complete, simultaneous discrimination between the 2 cell populations. Additionally, the covalent attachment of the dyes to proteins allows preservation of the live- and dead-cell population discrimination following fixation with formaldehye, under conditions that are commonly used for intracellular immunophenotyping and to inactivate pathogens.		Assay Principle Figure 1. Principle of LIVE/DEAD® Fixable Dead Cell Stain Kits.  Read More + Live cells (top) react with the kit’s fluorescent dye only on their surface to yield weak total fluorescence. Dead cells with damaged membranes (bottom) react with the dye on their surface as well as n the cell interior, yielding bright total fluorescence.