To directly assess the effect of UNG and SMUG1 on the generation of SBS2 and SBS13a/b in cancer cells, we expressed UNG–GFP in BC-1 cells, and CRISPR–Cas9 deleted SMUG1 from BT-474 cells and UNG from BT-474 and MDA-MB-453 cells (Extended Data Figs. 1k and 3a–c,g–j). The gene discussed is UNG; the disease is cancer.