Ethidium homodimer is a membrane-impermeant fluorescent dye that selectively stains nucleic acids in cells with compromised membranes. It is primarily used as an indicator of cell death in viability assays, penetrating only dead or dying cells to bind DNA and RNA, producing a red fluorescence upon excitation. Commonly paired with live-cell permeable stains (e.g., calcein-AM) for dual-color discrimination of live versus dead cells under fluorescence microscopy. It is also applied in flow cytometry for analyzing cell populations based on membrane permeability and in microbiology to evaluate membrane integrity. Unlike ethidium bromide, it is not typically used for routine DNA gel electrophoresis due to its larger size and higher specificity for dead cells.