Ten mitochondrial carriers have been purified from animal mitochondria. They are small proteins with a molecular mass ranging from 28 to 34 kDa on SDS-PAGE. So far, five of these proteins have been sequenced. Their polypeptide chain consists of three tandemly related sequences of about 100 amino acids. The repeats of the different proteins are related and probably fold into two transmembrane alpha-helices linked by an extra-membrane loop. The features of this family are also present in several proteins of unknown function characterized by DNA sequencing. Isoforms of some carriers have been found. All mitochondrial carriers investigated in proteoliposomes function according to a simultaneous (sequential) mechanism of transport. The only exception is the carnitine carrier that proceeds via a ping-pong mechanism. Three mitochondrial carriers have been expressed in yeast and two overexpressed in E. coli and refolded in active form.