LOV-domains are a sub-class of PAS domains and were first identified in plants as part of Phototropin, which plays an essential role in the plant's growth towards light. They noncovalently bind Flavin mononucleotide (FMN) as coenzyme. Due to the bound FMN LOV-domains exhibit an intrinsic fluorescence, which is however very weak. Upon illumination with blue light, LOV-domains undergo a photocyle, during which a covalent bond is formed between a conserved cysteine-residue and the FMN. This causes a conformational change in the protein that is necessary for signal propagation and also leads to the loss of fluorescence. The covalent bond is energetically unfavorable and is cleaved with a protein specific time constant ranging from seconds to hours.
In order to make better use of the fluorescence properties of these proteins, the natural photocycle of these LOV-domains was abolished by exchanging the conserved cysteine against an alanine on a genetic level.
Thus, upon blue light irradiation, the protein remains in the fluorescent state and also exhibits a brighter fluorescence.
The first FbFPs that were generated in this fashion were subsequently further optimized using different methods of mutagenesis. Especially the brightness  but also the photostability  of the proteins were enhanced and their spectral characteristics altered.