Conversely, however, A3 enzymes, in particular A3B, can mutate genomic DNA, especially in cancer cells, thereby contributing to cancer genome evolution, acquired drug resistance and poor survival prognosis in cases of multiple cancers (including breast, bladder, cervix, lung, head and neck).2 A3B inhibition thus presents a promising new strategy to complement existing anticancer therapies,3 because A3B is a nonessential protein.4 The gene discussed is APOBEC3B; the disease is cancer.