An ethidium homodimer assay can be used to detect dead or dying cells.
Ethidium homodimer is a membrane-impermeable fluorescent dye which binds to DNA.
After a cell sample has been stained with ethidium homodimer, the dead cells may be viewed and counted under a UV-light microscope.
Because live cells don't have a compromised membrane, the ethidium homodimer can't enter.
One reason for doing an ethidium homodimer assay instead of using a TUNEL assay to measure cell death is that ethidium homodimer stains all of the dead or dying cells, while TUNEL only stains cells that have undergone programmed cell death.