Mitochondrial intermembrane space import and assembly protein 40 (CHCHD4) is a chaperone and catalyses the formation of disulfide bonds in proteins within the mitochondrial intermembrane space (IMS) such as COX17 [], COX19 [], MICU1 []and GFER []. A precursor of one of these proteins in a reduced state is imported into the mitochondrion where it associates with CHCHD4 forming a transient intermolecular disulfide bridge. This leads to oxidation of the precursor, formation of an intramolecular disulfide bond and the folding in the IMS [].This entry also includes the uncharacterized protein F42H10.2 from Caenorhabditis elegans.