In the tumour samples, the majority of epithelial (tumour) cells expressed neither APOBEC3A nor APOBEC3B at levels detectable by scRNA-seq, and the only datasets containing a significant number of cells expressing APOBEC3A and/or APOBEC3B were from HNSCC or ESCC (Fig. 1). The gene discussed is APOBEC3B; the disease is head and neck squamous cell carcinoma.