The superb accuracy of transmission of genetic information between generations is one of the most fascinating properties of life. Infrequent errors in this transmission lead to mutations that are the source of genetic variation which fuels evolution and causes genetic disease. Most mutations generate individual single nucleotide substitutions but some are complex events such as substitution clusters. Mutation clusters contain more information than individual mutations and serve as a powerful tool to investigate mutational mechanisms using statistical methods. The analysis of more than 1 500 000 clustered mutations reveals three underling mutagenic processes. We attribute two of these processes to the translesion synthesis bypassing DNA damage.