Bimolecular fluorescence complementation

Bimolecular fluorescence complementation (BiFC) is a method of viewing the association of proteins inside living cells. Hu et al. (2002) solidified this methodology as viable in vivo.

How it works
The intact GFP protein (and its variants CFP, YFP, BFP, and RFP) is fluorescent, however, when the fluorescent protein is split into N and C-terminal halves, the molecule does not produce fluorescence. Fusing of each of the two non-fluorescent fragments to two putative interacting partners leads to restoration of fluorescence within a cell by reconstituting the split flurophore. This fluorescence is detected via fluorescence microscopy, which can be recorded by a mounted camera. The advantage of the BiFC method over other methods of visualizing protein-protein interactions is that it gives an indication of interaction, as well as cellular localization of the complex.

BiFC can be used as an alternative to FRET, or it can complement it.