Spontaneous apoptosis in the populations of the parental and RARα infected cell lines. Propidium iodide flow cytometry detection of dead cells was performed using the standard procedure. Cells were collected 24 h after seeding, washed with PBS, and fixed in 70% ethanol overnight at 4°C. Fixed cells were suspended in citric buffer and stained with 5 mcg/ml propidium iodide in PBS for 1 hour at 4°C. DNA content was subsequently measured by FACScan (Becton Dickinson, USA). All cells with sub-G0 DNA content were regarded as dead cells. This figure is representative of 3 separate experiments.