Techniques used to identify cells which have incorporated transfected DNA and which express the exogenous gene encoded by the transfected DNA are slow and often involve exposure of the cells to compounds which, as seen for example in biopharmaceutical production, are not always acceptable.  Fluorescent proteins are well-known and it has previously been shown that incorporation of a gene that expresses a fluorescent protein into a transfected DNA species allows cells expressing the fluorescent protein to be identified by flow cytometry.  It has also been shown that transfection of a DNA species in which a gene encoding a fluorescent protein is fused to a gene encoding a protein of interest, may be useful to localise the protein of interest in the transfected cells. 
However, the difficulty of screening for cells expressing two or more proteins encoded by transfected DNA when the proteins are not fused to a fluorophore remains.  The difficulties are further compounded when screening for expression of multi-subunit proteins. 
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.