DNA damage caused by ultraviolet light: 1. Cyclobutane-type pyrimidine dimer is the major photoproduct formed 2. A second product, 6-4 photoproduct, is formed in about 10 % of UV induced pyrimidine dimmers 3. Cyclobutane type dimer can be reversed by a process known as photoreactivation (a) this is carried out by an enzyme known as DNA photolyase (photoreactivating enzyme) (b) significance of photolyase enzyme in humans is questionable 4. DNA photoproducts can also be repaired by excision repair