Owing to its cytotoxicity, free copper is chelated by protein side chains and does not exist in vivo. Several chaperones transport copper to various cell compartments, but none have been identified that traffic copper to the nucleus. Copper-64 decays by β (+) and β (-) emission, allowing positron emission tomography and targeted radionuclide therapy for cancer. Because the delivery of (64)Cu to the cell nucleus may enhance the therapeutic effect of copper radiopharmaceuticals, elucidation of the pathway(s) involved in transporting copper to the tumor cell nucleus is important for optimizing treatment. We identified Atox1 as one of the proteins that binds copper in the nucleus. Mouse embryonic fibroblast cells, positive and negative for Atox1, were used to determine the role of Atox1 in (64)Cu transport to the nucleus. Mouse embryonic fibroblast Atox1(+/+) cells accumulated more (64)Cu in the nucleus than did Atox1(-/-) cells. HCT 116 colorectal cancer cells expressing p53 (+/+) and not expressing p53 (-/-) were used to evaluate the role of this tumor suppressor protein in (64)Cu transport. In cells treated with cisplatin, the uptake of (64)Cu in the nucleus of HCT 116 p53(+/+) cells was greater than that in HCT 116 p53(-/-) cells. Atox1 expression increased in HCT 116 p53(+/+) and p53(-/-) cells treated with cisplatin; however, Atox1 localized to the nuclei of p53(+/+) cells more than in the p53(-/-) cells. The data presented here indicate that Atox1 is involved in copper transport to the nucleus, and cisplatin affects nuclear transport of (64)Cu in HCT 116 cells by upregulating the expression and the nuclear localization of Atox1.