Using two human breast epithelial cell lines, MCF10A (non-tumorigenic, immortalized mammary epithelial cell line) and MCF7 (tumorigenic), that have borderline undetectable endogenous A3 expression (Burns et al., 2013a), we developed stably transduced cell lines expressing either Flag-tagged A3A, A3B, or A3H Hap I and tested the induced DNA damage, deamination activity, and ability to induce cancer cell phenotypes. Here, APOBEC3H is linked to cancer.