Members of the mitochondrial carrier family transport compounds over the inner mitochondrial membrane to link the biochemical pathways in the cytosol with those in the mitochondrial matrix. X-ray crystallography has recently provided us with the first atomic model of the bovine ADP/ATP carrier, which is a member of this family. The structure explains the typical three-fold sequence repeats and signature motif of mitochondrial carriers. However, the carrier was crystallised as a monomer in detergent, which is inconsistent with the consensus that mitochondrial carriers exist as homo-dimers. The projection structure of the yeast ADP/ATP carrier by electron crystallography shows that carriers could form homo-dimers in the membrane.