In order to investigate the function and regulation of defined proteins – such as by pharmaceutical compounds or feed additives – candidate genes can be overexpressed in cell culture systems and investigated. In order to investigate a particular ion channel, it can be useful to insert the associated gene into a vector. For control purposes, it is possible to simultaneously insert a marker such as a gene for a fluorescent protein (here: GFP). After synthesis, the vector is introduced into the cell and transcribed by the cellular protein expression system. Successfully transfected cells not only express the channel of interest, but also the marker protein. In the case of GFP, transfected cells emit a green fluorescent signal and can be selectively investigated, i.e. via patch clamping.