The eBioscience Fixable Viability Dye is an amine-reactive dye and covalently binds to primary amines, such as those present on proteins. Amine-reactive dyes cross the cell membrane of dead cells, and react with free amines in the cytoplasm. Live cells exclude the dye because their cell membranes are intact. Samples that are stained with this viability dye show a bright and dim population. The very bright cells are dead, while the dimmer population represents live cells. This is because the dye can bind to all amines (extracellular and intracellular) in dead cells, whereas it can only stain surface amines in viable cells.