The exchange of 13C label between pyruvate and lactate in a tumor is dependent on [1‐13C]pyruvate delivery, cell uptake via the monocarboxylate transporters 1 and 4 (MCT1 and MCT4), the concentrations of lactate dehydrogenase and its coenzymes, and the size of the endogenous tumor lactate pool.30 The gene discussed is SLC16A1; the disease is neoplasm.