The fact that we only observed high APOBEC3A expression in a small minority of S-phase cells in our HNSCC samples is consistent with the proposed episodic nature of APOBEC-mediated mutagenesis, in which the chances of observing a mutagenic burst in the snapshot provided by a tumour biopsy are low (Petljak et al, 2019, 2022a, 2022b; Smith and Fenton, 2019). The gene discussed is APOBEC3A; the disease is head and neck squamous cell carcinoma.