AICDA and esophageal squamous cell carcinoma: Finally, the “Background” signature represents the base level mutational processes in ESCC which correlates strongly to COSMIC signature 3 (the failure of double-strand-break repair by homologous recombination; CS = 0.93), signature 4 (tobacco exposure; CS = 0.70), signature 9 (the activity of AID during somatic hypermutation; CS = 0.70) and several other signatures of unknown background (COMIC signature 5, 8, 16, 25) [13].