In breast cancer cells transfected with human papillomavirus type 18, A3B overexpression was accompanied by increased genomic instability (demonstrated by DNA comet assay); this effect could be inhibited by small interfering RNA targeting HPV18 E6 and E7, as well as A3B (gene knockdown confirmed by reverse transcription quantitative PCR). The gene discussed is APOBEC3B; the disease is breast carcinoma.