The original immunofluorescence technique introduced by AH COONS and co-workers about 60 years ago has been refined during the last two decades in a manifold way. New developments such as confocal, deconvolution, ratio-imaging, total internal reflection and applications of fluorescent reporter molecules (such as green fluorescent protein [GFP] and variants, briefly called FP for fluorescent proteins) have now become integral tools in functional genomics and cell development. In functional genomics, transgenic approches using FPs might well be exploited to examine promoter activity and to clone regulatory elements.