Mutational processes and chromatin state. (A) Comparison of the fraction of mutational opportunity in each chromatin state portion of the human genome to the fraction of actual somatic mutations found in them (red dots). The numbers at the top show the mean over- or under-representation of somatic mutations per chromatin state. (B) Of the mutational processes identified in breast cancer, process C shows the strongest variation with chromatin state. In promoter regions, process C mutations are suppressed by 80% on average, whereas in heterochromatin regions there is, on average, a 40% surplus of them. Green: process A; pink: process B; blue: process C; yellow: process D.