Mohr-Tranebjaerg syndrome (MTS/DFN-1) or deafness/dystonia syndrome results from a mutation in deafness/dystonia protein 1/translocase of mitochondrial inner membrane 8a (DDP1/TIMM8a). DDP1/TIMM8a is similar to a family of yeast proteins in the mitochondrial intermembrane space which mediate the import and insertion of inner membrane proteins. We now show that TIMM8a assembles in a 70 kDa complex in the intermembrane space with TIMM13. DDP1/TIMM8a is not detectable in fibroblasts derived from a patient with a missense mutation in the DDP1/TIMM8a gene; the point mutation results in cysteine-66 being changed to tryptophan-66 in the conserved 'twin CX(3)C' motif. The corresponding mutation in yeast translocase of inner membrane 8p (Tim8p) yields an unstable protein that does not assemble with yeast Tim13p. DDP1/TIMM8a, when expressed with TIMM13 in yeast mitochondria lacking the Tim8p-Tim13p complex, restores Tim23p import, and TIMM8a and TIMM13 can be cross-linked to the hTim23 import intermediate in rat and yeast mitochondria. In a similar manner to Tim8p, TIMM8a seemingly mediates the import of hTim23. Deafness/dystonia syndrome thus may be caused by decreased levels of Tim23 in the mitochondrial inner membrane in affected tissues.