SLC25A1, in the inner mitochondrial membrane, mediates the exchange of cytosolic citrate for mitochondrial phosphoenolpyruvate. The exchange is physiologically irreversible because of the potential of the inner mitochondrial membrane. The gene encoding human SLC25A1 has been cloned and its expression pattern has been characterized (Heisterkamp et al. 1995; Iacobazzi et al. 1997), but the biochemical details of the transport process are inferred from those worked out for the well-characterized rat system (Robinson 1971; Soling et al. 1971; Kleineke et al. 1973).