Tranebjaerg et al2 reported that the Norwegian pedigree showed a significant linkage disequilibrium to Xq21.3-q22. The Bruton gammaglobulinemia tyrosine kinase (BTK) gene, which is involved in immunodeficiency X-linked agammaglobulinemia,6 is located in this region. Jin et al3 analyzed the X-linked agammaglobulinemia patients who had deafness and dystonia, and found that deletion of the BTK gene extended to the next gene in the 3′ location of the BTK gene. They designated this gene as DDP (deafness-dystonia peptide) and cloned the transcript lying in that region. The DDP gene has 2 exons and a single intron of about 2 kb and encodes a small peptide consisting of 97 amino acids. Jin et al found 2 kinds of microdeletion in the coding region of the DDP gene in a Norwegian patient and an American patient. In the Norwegian patient, a 1-bp deletion (151delT) in exon 1 produced a consequent frame shift resulting in an incorporation of 25 novel amino acids after glutamic acid at codon 38, followed by early termination. In the American patient (pedigree K8190), a 10-bp deletion (183del10) in exon 2 of the DDP gene produced a frame shift resulting in the addition of 12 novel amino acids after the methionine residue at codon 48, followed by early termination. In our cases, arginine at codon 80 was substituted by a stop codon. Therefore, this nonsense mutation produced a truncated DDP gene of 79 amino acids. However, this was longer than the normalamino acid sequence of those peptides produced by the microdeletions (151delT and 183del10) of the DDP gene found by Jin et al3; normal sequences of these have only 38 and 48 amino acids, respectively. It is possible that the truncated peptide produced by the nonsense mutation with a longer normal amino acid sequence could be related to the milder clinical symptoms and slower progression of the disease seen in the Japanese patients with X-linked DDS. The physiological function of the DDP gene is unknown. Although it is expressed at the highest concentration in the fetal brain, it is expressed ubiquitously in other tissues, such as the liver, heart, kidney, and lung.7 Recently, DDP was shown to strongly resemble Tim8p, a zinc-binding yeast protein that is implicated in the import of a class of transmembrane carrier proteins from the cytoplasm to the mitochondrial inner membrane.7,8 It was also found that DDP protein is located in the mitochondrial intermembrane space.8 Therefore, mutated DDP may disrupt the mitochondrial import system and energy production, inducing dystonia and deafness.