Does propidium iodide stain apoptotic cells?

Propidium iodide (PI) will not be able to stain early apoptotic cells due to the presence of an intact cell membrane. Late apoptotic cells, which are considered dead cells, can be stained by propidium iodide due to not having a cell membrane. It all connects to membrane permeability; PI will not stain cells that are protected by a cell membrane.