A growing number of mutations in mitochondrial (mt) tRNA genes have been found to associate with human mitochondrial diseases. Our previous analysis of mutant mt tRNAs isolated from cells derived from patients with mitochondrial diseases revealed the lack of a post-transcriptional taurine-modification at the anticodon wobble uridine in two mt tRNAs bearing typical pathogenic mutations: mt tRNA Leu(UUR) with either the MELAS 3243 or 3271 mutation and mt tRNA Lys with the MERRF 8344 mutation. We here summarize our recent studies that clarify the molecular basis of the defective mitochondrial translation caused by this wobble modification deficiency. The MERRF mt tRNA Lys lacking the wobble modification cannot translate either of its codons (AAA and AAG), while the translational activity of MELAS mt tRNA Leu(UUR) lacking wobble modification is more depressed in decoding of UUG codon than UUA codon. These findings suggest that the wobble modification deficiency plays a primary role in the molecular pathogenesis of the MELAS and MERRF mitochondrial diseases.Point mutations as well as large scale deletions in mitochondrial DNA (mtDNA) are associated with a wide spectrum of human diseases arising from mitochondrial dysfunction. 1,2 Over 150 different pathogenic mutations have been reported so far, more than half of which are located within mt tRNA genes. 1,2 The clinical features of mitochondrial diseases often depend on the mt tRNA species and/or the positions of the mutations involved. However, it is not clear how, at the molecular level, the mutant mt tRNAs actually cause mitochondrial dysfunction and specify the clinical features they generate. If a pathogenic mutation in the mt tRNA gene does not affect mtDNA replication or transcription of the corresponding mt tRNA, it can be assumed that its deleterious effect is exerted at the post-transcriptional level. In other words, it may affect mt tRNA maturation, modification, folding, or aminoacylation, its association with translation factors, and/or various functions on the ribosome. To elucidate the essential molecular pathogenesis of mitochondrial diseases, detailed structural and functional analyses of the mutant mt tRNAs are thus required.We have recently found that of the various post-transcriptional events that an mt tRNA mutation could potentially impair, some mutant mt tRNAs fail to undergo particular post-transcriptional modifications. We previously identified two novel taurine-containing modified uridines [5-taurinemethyluridine (τm 5 U) and 5-taurinomethyl2-thiouridine (τm 5 s 2 U)] at the anticodon first (wobble) position of some species of mammalian mt tRNAs (Fig. 1). 3 What is more, these taurine-containing modifications were lacking in the mutant mt tRNAs from two typical mitochondrial diseases. Our additional analyses suggest that the absence of these modifications can be the main cause of the mitochondrial dysfunction associated with these diseases.