Abstract Pyrimidine dimer is an intrastrand DNA cross-link, induced by exposure to ultraviolet light (sunlight). Two types of dimers are formed, depending on whether DNA is single-stranded or duplex. Pyrimidine dimers block both DNA replication and transcription and have to be removed to return DNA to its functional state. Multiple DNA repair mechanisms are known to remove pyrimidine dimers and individual organisms usually have at least two different ones. In humans, defects in pyrimidine dimer removal cause disease and cancer.