Cyclobutane pyrimidine dimers (Pyr < > Pyr) are the major DNA photoproducts induced by the UV component of solar radiation. Photoreactivating enzyme (DNA photolyase) repairs DNA by utilizing the energy of visible light to break the cyclobutane ring of the dimer. Photolyases are monomeric proteins of 50-60 kDa with stoichiometric amounts of two noncovalent chromophore/cofactors. One of these cofactors is FADH-, and the second chromophore is either methenyltetrahydrofolate (MTHF) or 8-hydroxy-5-deazariboflavin (8-HDF). The enzyme binds the DNA substrate in a light-independent reaction, the second chromophore of the bound enzyme absorbs a visible photon and, by dipole-dipole interaction, transfers energy to FADH- which, in turn, transfers an electron to Pyr < > Pyr in DNA; the Pyr < > Pyr- splits and back electron transfer restores the dipyrimidine and the functional form of flavin ready for a new cycle of catalysis.