text: Due to its relatively short turnover time of ~6 weeks in the human body, Cu serves as an important indicator of cancer and other diseases that rapidly evolve. The serum of cancer patients contains significantly higher levels of Cu than that of healthy patients due to copper chelation by lactate, which is produced via anaerobic glycolysis by tumor cells. These imbalances in Cu homeostasis are reflected isotopically in the serum and organ tissues of patients with various types of cancer, where the serum of cancer patients is generally Cu-depleted relative to the serum of healthy patients, while organ tumors are generally Cu-enriched. In one study, the blood components of patients with hepatocellular carcinomas (HCC) was found to be, on average, depleted in Cu by 0.4‰ relative to the blood of non-cancer patients. In particular, the δCu values of the serum in patients with HCC ranged from -0.66 to +0.47‰ (compared to serum δCu values of -0.39 to +0.38‰ in matched control patients), and the δCu values of the erythrocytes in the HCC patients ranged from -0.07 to +0.92‰ (compared to erythrocyte δCu values of +0.57 to +1.24‰ in matched control patients). The liver tumor tissues in the HCC patients were Cu-enriched relative to healthy liver tissue in the same patients (δCu = -0.02 to +0.43‰; δCu = -0.45 to -0.11‰), and the magnitude of Cu-enrichment mirrored that of the Cu-depletion observed in the cancer patients' serum. Though our understanding of how copper isotopes are fractionated during cancer physiologies is still limited, it is clear that copper isotope ratios may serve as a powerful biomarker of cancer presence and progression.