Insect cells transfected with heterologous DNA can be used to evaluate the functional properties of genes that have been isolated or subjected to in vitro mutagenesis. An example of such a cell system is embryonic cells from Drosophila melanogaster. These can be transfected with special plasmids and used for heterologous protein expression. FISH is an elegant technique for controlling transfection efficiency at the single-cell level. The background and procedure are described in detail here.