The Goldilocks expression levels of human A3B enabled by the CAG promoter (after Cre-mediated excision of the transcription stop cassette) are likely to be helpful for studying the formation, evolution, and therapy responsiveness of many different APOBEC signature-high tumor types including those of the bladder, breast, cervix, head/neck, lung, and other tissue types. The gene discussed is APOBEC3B; the disease is neoplasm.