It depends on your experiment. For instance, say you had your cells treated with a drug that stimulated cells to proliferate for a short period of time. Then you performed your staining, so your S region would increase as much as G1 peak decrease.
In another situation, say a drug treatment that arrests cells in M phase (colcemid or mitomycin) if you wait a short time you would be in a similar scenario as the one descibed above. However if you wait enough, your G2M peak will increase until no G1 or S populations would be detactable.
This is a difficult question. To answer it, it would be good to know something about the type of cells, the treatment and the sampling for FCM, the protocol used to stain for nuclear DNA content. Assuming that you are staining cells in the context for their DNA content and that you measure intact cells for their DNA content, you will have the following options. If you just stimulate the prolifereation by increasing S-phase, this will temporarily increase G2/M phase, provided your sampling frequency is high enough. If you want to get an idea of the increase in DNA synthesising cells, you could add mitotic inhibitors such as colchemid, colchicine for animal cells and APM or oryzalin for plant cells at varing times after induction of the profileration. Since cells treated with these chemicals do not divide their chromosomes, they will end up as double DNA content nuclei in your FCM analysis. Experimental setup is very important in this case to have the proper control without treatment. If you are interested, I can provide you with a more detailed scheme how to tackle this problem. Kind regards, Harrie Verhoeven.