The Green Fluorescent Protein (GFP) is a spontaneously fluorescent polypeptide of 27 kD from the jellyfish Aequorea victoria that absorbs UV-blue light and emits in the green region of the spectrum. GFP has been successfully expressed both in bacteria and in eukaryotic cells and is widely used to monitor the localization of tagged proteins in living cells. Since wtGFP performs inefficiently in different cellular contexts, efforts have been devoted to the improvement of GFP expression levels and/or fluorescence. We will here review the basic characteristics of wt and mutated GFP, in particular their protein expression vs fluorescent properties. Emphasis will be given to unexpected consequences of mutations of the GFP gene, i.e. on transcription and translation rates and on protein folding in different cell types, and to how these critically reflect on the use of GFP in different cellular environments.