The Cambronne lab studies how metabolites are regulated in different parts of the cell to control signaling pathways and gene expression. We develop genetically-encoded fluorescent biosensors for localized and real-time measurements of these metabolites. Our initial work has focused on oxidized nicotinamide adenine dinucleotide (NAD+) for its dual roles in oxidoreductive reactions and as the substrate for NAD+ consuming signaling enzymes. A mitochondrial NAD+ sensor contributed to de-orphaning mitochondrial carrier SLC25A51 as the major transporter of free NAD+ in humans. We found that the selectivity of SLC25A51 for oxidized NAD + can be attributed to charged interactions between the nicotinamide ring and an acidic patch in the central pore of SLC25A51. With its selectivity for oxidized NAD + , SLC25A51 serves as a decoupler for NAD + /NADH ratios in mitochondria, and as such, a promising vulnerability in oxidative cancers such as drug-refractory acute myeloid leukemias. Ongoing studies include understanding how electrogenic properties of SLC25A51 influence its directionality of transport.