In HNSCC, we find evidence of GRHL3 activity and APOBEC3A in a subpopulation of tumour cells undergoing DNA replication; the context in which mutagenic APOBEC activity is postulated to occur due to deamination of lagging strand ssDNA exposed at the replication fork (Green et al, 2016; Haradhvala et al, 2016; Hoopes et al, 2016; Morganella et al, 2016; Seplyarskiy et al, 2016; Stewart et al, 2020). This evidence concerns the gene GRHL3 and head and neck squamous cell carcinoma.