Photolyase is an enzyme that binds complementary DNA strands and breaks pyrimidine dimers that are typically caused by exposure to ultraviolet light. Pyrimidine dimers occur when a pair of thymine bases or cytosine bases on the same strand of DNA bind together, resulting in a 'bulge' of the DNA structure referred to as a lesion. Photolyase has a high affinity for these lesions in the DNA and will reversibly bind and split the dimer using light-energy.