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> Two plasmids bearing same origin of replication will not co-exist in > the cell for very long * although the above is correct, using plasmids that are in principle not * compatible * but have different resistance to antibiotics, works well in a number of * cases and is easy to do. Yes, I agree - this often does work for a while - which is why I wrote that the two plasmids won't coexist for very long. So if you're only interested in quick-and-dirty screening, you might want to try this. I would not recommend this method for any work that requires long-term stability and reproducibility of expression - the two plasmids of the same origin, that bear different dominant selection markers, will eventually recombine into one or more new plasmids (concatemers or recombinants bearing both markers) and it's hard to say what will happen with respect to expression. Artem
