text: Zn isotopes may be useful as a tracer for breast cancer. Relative to non-cancerous patients, breast cancer patients are known to have significantly higher concentrations of Zn in their breast tissue, but lower concentrations in their blood serum and erythrocytes, due to overexpression of Zn transporters in breast cancer cells. Consistent with these body-wide shifts in Zn homeostasis, δZn values in breast cancer tumors of 5 patients were found to be anomalously light (varying from -0.9 to -0.6‰) relative to healthy  tissue in 3 breast cancer patients and 1 healthy control (δZn = -0.5 to -0.3‰). In this study, δZn values of blood and serum were not found to be significantly different between cancerous and non-cancerous patients, suggesting an unknown isotopically heavy pool of Zn must exist in cancer patients. Though results from this study are promising regarding the use of Zn isotope ratios as a biomarker for breast cancer, a mechanistic understanding of how Zn isotopes fractionate during tumor formation in breast cancer is still lacking. Fortunately, increasing attention is being devoted to the use of stable metal isotopes as tracers of cancer and other diseases, and the usefulness of these isotope systems in medical applications will become more apparent in the next few decades.