Cell cycle analysis after viral transduction - Shift of peak (Feb/13/2011 )

I am currently doing some cell cycle study using flow/PI staining

more specifically, I want to know the effect of certain gene expression to the certain cell in thier cell cycle in vitro

I have experience with cell cycle analyis using PI staining/flow so the question is not really technical issue, but
I just oberved quite unexpected things and I do not know how that happen, so I would like to know you guys opinion.

so for my experiment, I made two retro virus, one is control virus (just GFP), other is GFP with gene of interest.

initially I wanted to compare these two only, but I also did PI staining the cell without viral transduction.

and what I observed is that when the cell is transduced even with control virus, there is a shift in peaks (Go/G1 and G2/M)
I was initially expected just in/decrease of certain peak, but the % of each peak does not have much change,
instead entire peak moved to left (in PI Area)for the viral transduced cell (both control virus and experimental virus)
also this is not just transduced cell, even GFP-ve cells in transduced cell population had shifted peaks

not quite sure why???? does anyone have any idea???
maybe polybrane that I used for the transdcution affects somehow?
if anyone have some idea, let me know

Thanks

Hmm...Is the compensation between GFP and PI is correct? Check it.