Good, but in those system you always got mixed response from individual cells. ie, in a Tet-On system your expressed protein level will be varied in cells if you investigated at microscopic level.

That's what I thought, thanks.

You can also do a stable modification of cells - if You use non-homologus recombination You would have both: 1. different copy-number of integrated construct as a head-to-tail fusions which will give different level of expression as measured by RealTime-PCR (in a limited range, as when there is more than 10 copies, the cell can do intrachromosomal recombination and eliminate inserted constructs), 2. position effect - the surrounding regulatory sequences will modulate expression of Your construct. This way You will be able to obtain clones with the different levels of expression of Your chosen protein. Of course, there could be some problem with insertional mutagenesis but it is only at 10% level max.