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Betel quid (BQ) chewing is a prevailing risk for oral squamous cell carcinoma (OSCC) in Southeast Asia. Yet, the detailed mechanisms by which BQ chewing damages the genome are still not fully understood. Through exome sequencing of tumor–normal pairs from 196 male patients with OSCC, including 95 habitual BQ chewers and 101 non-BQ users, we conducted a quantitative survey of mutational signatures and genomic aberrations and explored their association with BQ chewing. We found that BQ-associated elevation in mutation rate was seen in cancers of the tongue, but not in overall OSCC. Additionally, we identified a mutational signature that is enriched in tumors from BQ users. Moreover, the numbers of small insertions and deletions (INDELs) and breakpoints derived from structural variations (SV) were increased, whereas the extent of loss of heterozygosity was decreased in BQ-related OSCC genomes. However, neither the number of base substitutions and microsatellite instability events nor the extent of copy-number alterations differed between BQ-related and -unrelated OSCC. In conclusion, consistent with the proposition that BQ chewing increases OSCC risk as a mutagen, our results unveil a BQ-associated mutational signature and indicate mutagenic impacts of BQ chewing on preferentially eliciting INDELs and SV-related breakpoints in OSCC genomes. [ABSTRACT FROM AUTHOR]