We have recently identified a subfamily of mitochondrial carriers that bind calcium, and cloned ARALAR1, a member of this subfamily expressed in human muscle and brain. We have now cloned a second human ARALAR gene (ARALAR2) coding for a protein 78.3% identical to Aralar1, but expressed in liver and non-excitable tissues. Aralar2 is identical to citrin, the product of the gene mutated in type-II citrullinaemia [Kobayashi, Sinasac, Iijima, Boright, Begum, Lee, Yasuda, Ikeda, Hirano, Terazono et al. (1999) Nat. Genet. 22, 159-163]. A related protein, DmAralar, 69% identical to Aralar1, was found in Drosophila melanogaster, the DMARALAR locus lying on the right arm of the third chromosome, band 99F. The N-terminal half of Aralar2/citrin is able to bind calcium and this requires the presence of the two most distal EF-hands. The localization of Aralar2/citrin expressed in human cell lines is mitochondrial, the C-terminal half containing sufficient information for import and assembly into mitochondria. The C-terminal half of Aralar proteins is related to the yeast YPR020c gene, with a very high sequence conservation (54.3% identity), suggesting that these proteins play an important role. Thus Aralar proteins are probably expressed in all tissues in an isoform-specific fashion, where they function as calcium-regulated metabolite (possibly anionic) carriers.