One such possible signal transduction mechanism involves infection with human papillomavirus (HPV), i.e., the HPV strain HPV16 or HPV18 induces A3B expression in cultured cells of breast and head/neck cancers, and the virus-encoded protein E6 directly binds the proximal A3B promoter and triggers transcription; however, the prevalence of HPV involvement in cancers is not fully known18, 19. The gene discussed is APOBEC3B; the disease is cancer.