The APOBEC proteins are DNA cytosine deaminases with roles in immunity, including retroviral restriction and antibody maturation. Their activity theoretically makes them a danger to genomic DNA. The subfamily of APOBEC3 genes has expanded to included seven different genes in primates. Based on their subcellular localization, only a subset of these APOBEC3 proteins have access to genomic DNA, and may potentially deaminate genomic DNA. Although the nuclear envelope breaks down during mitosis, I demonstrate that none of the APOBEC3s gain access to genomic DNA during cell division. However, APOBEC3B and other APOBEC3 proteins have access to genomic DNA during interphase. I also show that APOBEC3B is actively imported into the nuclear compartment. In general, APOBEC3 nuclear localization and deaminase activity correlate with ability to affect cell cycle progression, implicating these APOBEC3s in deamination of genomic DNA. In support of these conclusions, I observed cell death, activation of the DNA damage response and DNA mutations after ectopic expression of APOBEC3A and APOBEC3B. Moreover, endogenous APOBEC3B is demonstrably nuclear and active in breast cancer cell lines where it causes genomic deamination and mutations. Endogenous APOBEC3B is highly expressed in more than half of human breast cancers compared to normal breast tissues. In addition, sequences from tumors with higher levels of APOBEC3B have more mutations, and these mutations match APOBEC3B's deamination signature. My thesis work further defines the subcellular localization of the APOBEC3 family and provides the first evidence that APOBEC3B is involved in a human cancer type.