 Translesion polymerases frequently introduce mutations at pyrimidine dimers, both in prokaryotes (SOS mutagenesis) and in eukaryotes.Although the thymine-thymine CPDs (thymine dimers) are the most frequent lesions caused by UV light, translesion polymerases are biased toward introduction of As, so that TT dimers are often replicated correctly.On the other hand, any C involved in CPDs is prone to be deaminated, inducing a C to T transition.