In Mlh1−/− cells — where replication errors are left unrepaired — mononucleotide deletions were substantially more common than insertions (Figures 1B,C), which is in line with previous findings in MMR-deficient tumor genomes (Kondelin et al., 2017) and implies that microsatellite sequences are much more prone to template-strand loops compared to nascent-strand loops. Here, MLH1 is linked to neoplasm.