The mitochondrial disulfide relay system facilitates the import and folding of a subset of soluble proteins into mitochondrial intermembrane space from the cytosol. The majority of substrate proteins contain conserved cysteine (twin-CX9) motifs. These cysteines are oxidized to disulfide bonds by Mia40 during the import process. (Adapted from FBrf0246713 and PMID:37159021.)