Calcium-binding mitochondrial carrier proteins Aralar1 and Aralar2 (SLC25A12 and SLC25A13 respectively), located in the inner mitochondrial membrane, mediate the exchange of cytosolic aspartate and mitochondrial glutamate (Palmieri et al. 2001). The exchange is physiologically irreversible because of the potential across the inner mitochondrial membrane (positive outside, negative inside). In the body, SLC25A12 is found mainly in heart, skeletal muscle, and brain, while SCL25A13 is widely expressed but most abundant in liver (del Arco et al. 2000; Palmieri et al. 2001). Defects in SLC25A13 are associated with type II citrullinemia, characterised by a liver-specific deficiency of the urea cycle enzyme argininosuccinate synthase (Kobayashi et al. 1999, Saheki et al. 2002).