I am very confused about single molecule FRET. My concern is for very weak dimers that follow law of mass action, people have performed single molecule FRET (for example EGFR). I am not understanding if there is one molecule and it is not a constitutive dimer the chances that it will ever be a dimer is almost zero. So how can one perform single molecule FRET. Any expert's explaination on this subject will be appreciated.