Required for the import and folding of small cysteine-containing proteins (small Tim) in the mitochondrial intermembrane space (IMS). Forms a redox cycle with ERV1 that involves a disulfide relay system. Precursor proteins to be imported into the IMS are translocated in their reduced form into the mitochondria. The oxidized form of MIA40 forms a transient intermolecular disulfide bridge with the reduced precursor protein, resulting in oxidation of the precursor protein that now contains an intramolecular disulfide bond and is able to undergo folding in the IMS (By similarity). Cu(2+) or Zn(2+). Monomer. The CHCH domain contains a conserved twin Cys-X(9)-Cys motif which is required for import and stability of MIA40 in mitochondria. The predicted gene AN6694 has been split into 2 genes: AN6694 and AN11898.