SLC25A15 and SLC25A2, mitochondrial ornithine transporters, mediate the exchange of cytosolic ornithine for mitochondrial citrulline across the inner mitochondrial membrane. Both human proteins have been experimentally characterized (Fiermonte et al. 2003). DNA sequences capable of encoding chicken orthologs of both have been computationally identified in the ENSEMBL chicken gene set, although it is not clear which chicken gene corresponds to which human one. The chicken reaction is inferred from the human one although, given the evidence that citrulline is not synthesized from ornithine to an appreciable extent in chickens (Tamir and Ratner 1963), the physiological significance of the reaction is unclear.