apoptosis - (Sep/05/2009 )

I'am doing a time course for apoptosis......with my drug I can see caspase, parp cleavage and modulation of other apoptotic markers in as early as 6hrs but when I do apoptotic staining with AnnV/PI stain I can see no staining in 6,12hrs very little AnnV /PI staining (5%) in 24hrs and around 20 % in 36hrs.............why is it so since I see the proteins modulated in as early as 6hrs.............Also can some one let me know what is the sequence of apoptotic events specially at what point PS exposure occurs

As far as I remember the major events are (in order)

<*>PS exposure
<*>Caspases
<*>Cytochrome C
<*>DNA cleavage