Propidium iodide, a cell-membrane impermeable dye, is frequently used to stain DNA, with an excitation maximum at 535 nm and an emission maximum at 617 nm. Although propidium iodide is generally excluded from viable cells with intact membranes, it can easily penetrate dead or damaged cells, which makes it commonly used for identifying cell viability in a population or as a counterstain in multicolor fluorescent techniques. Propidium iodide intercalates with nucleic acids with a stoichiometry of one dye per 4 ~ 5 base pairs with little sequence preference, and is then detected by fluorescence microscopy and flow cytometry. Propidium iodide has been shown to be a useful tool to assess apoptosis of certain cell populations in heterogeneous tissues including bone marrow, lymph nodes and thymus. 

References:

1. Coder DM. Assessment of cell viability. Current Protocols in Cytometry, 2001, Chapter 9: Unit 9.2.

2. Hezel M, Ebrahimi F, Koch M, et al. Propidium iodide staining: a new application in fluorescence microscopy for analysis of cytoarchitecture in adult and developing rodent brain. Micron, 2012, 43(10): 1031-1038.

3. Nicoletti I, Migliorati G, Pagliacci MC, et al. A rapid and simple method for measuring thymocyte apoptosis by propidium iodide staining and flow cytometry. Journal of Immunological Methods, 1991, 139(2): 271-279.