In MCF10A normal-like breast epithelial cells and MCF7 breast cancer cells, which both express low levels of A3B (Burns et al., 2013a), this construct supported modest levels of transcription activity above those of a promoter-less vector (compare black bars of pGL3-basic versus pA3B-luciferase in Figure 1B). Here, APOBEC3B is linked to breast cancer.