The ability of glucocorticoids to induce apoptosis or programmed cell death in 
mouse thymocytes is well-established. Measurement of apoptosis-associated 
internucleosomal DNA fragmentation through determination of the percentage of 
fragmented DNA by electrophoresis or centrifugation of whole cell lysates is by 
far the most common means of quantifying apoptosis. Since these methods measure 
DNA fragmentation in whole cell lysates rather than intact cells, they have 
severe limitations, particularly with heterogeneous cell populations. When mouse 
thymocytes were incubated with glucocorticoids, fixed, stained with propidium 
iodide and analysed flow cytometrically for cell cycle distribution, a distinct 
subpopulation of cells was observed to form below the G0/G1 region, denoted as 
the A0 region. The presence of cells in this region was consistent with the 
presence of internucleosomal DNA fragments as determined by gel electrophoresis. 
Inhibitors of transcription, translation and endonuclease activity, and a 
glucocorticoid receptor antagonist prevented accumulation of cells in this 
region. Irradiation of mouse thymocytes also produced a population in the A0 
region. Cells in this region are believed to have undergone 
glucocorticoid-induced DNA fragmentation. This method represents a useful 
alternative to whole cell lysate assays, since apoptosis can be evaluated on an 
individual cell basis.