The robust APOBEC3A/B response to BKPyV infections in bystander cells gives urothelial carcinogenesis a distinctive mechanism, with the mutations occurring in trans to the infection: “transmutagenesis.” Carcinogenesis by episomal BKPyV infections contrasts with tumors where the viral genome is integrated into the initiated cell and the APOBEC3A/B mutations (12, 42–44) occur in cis (45) (hereafter “cismutagenesis”). Here, APOBEC3A is linked to infection.