Cyclobutyl dimer

A cyclobutyl dimer links two molecules or regions of molecules by a four-carbon ring. Such a ring is commonly formed between adjacent pyrimidine nucleotides in DNA that has been exposed to ultraviolet light. It is especially common between thymine bases, in which case it is known as a thymine dimer and repaired by the cell's DNA repair pathway. The resultant pyrimidine dimer produced causes a block to essential DNA replication and hence must be repaired for the cell to survive. Xeroderma pigmentosum is a genetic disease where this damage cant be repaired resulting in skin discolouration and multiple tumours on exposure to UV light.