Insect cells transfected with heterologous DNA can be used to assess 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 derived from Drosophila melanogaster . These cells can be transfected with special plasmids and used for heterologous protein expression. FISH is an elegant technique for monitoring the transfection efficiency at the single-cell level. The background of this method and the approach used are described here in detail.