Two proteins of interest are labelled with fluorophores that form a FRET pair. One is a donor fluorophore (green) and the other is an acceptor (red). Each fluorophore has an absorption spectrum (shown on the left side in each colour) and an emission spectrum (shown on the right side in each colour). If the proteins are in close physical proximity, exciting the donor fluorophore will enable transfer of energy to the acceptor fluorophore, exciting it in turn, and enabling the interaction to be measured as emitted light from the acceptor fluorophore.