2-aminopurine has been used as a fluorescent analogue of adenine for many years.Its fluorescence is strongly quenched when incorporated within DNA and perturbations such as base flipping can cause a dramatic enhancement of the emission.However,it has been found that not all methyltransferases cause such an enhancement even though base flipping should be occurring.Some enzymes even cause an enhancement when the 2-AP is placed at a site which should not undergo base flipping.