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Genetic diagnostics have undergone a revolution in the last decade, fueled by technological advances heralded by the development of massively parallel DNA sequencing. Within a remarkably short time, the new chemistry moved from the research laboratory into clinical practice, accelerating the pace of gene discovery and allowing the rapid diagnosis of genetic disorders on an unprecedented scale. There is a strong argument that so-called next or third generation sequencing will have the greatest effect in neurology, which is characterized by a seemingly endless list of discrete clinical syndromes, many thought to have a unique genetic basis. Until 2011, clinical neurogenetic practice has been frustrating. It has been difficult to screen more than a handful of the known genetic causes of a particular disorder, but we now face the real prospect of a reaching genetic diagnosis for every patient walking to the clinical door. Mitochondrial disorders provide a good illustration of the effect of this new technology in neurogenetic practice. Mitochondrial disorders are the bane of the generalist. The myriad of clinical features overlap with common neurological and nonneurological diseases. Despite entering the differential diagnosis of the most common neurological diseases, mitochondrial disorders themselves are rare, and the diagnostic approach is complex, time consuming, and expensive. Following a detailed history, examination, and clinical investigations, many patients require an invasive biopsy of clinically affected tissue before samples are dispatched on ice to a limited number of laboratories worldwide. Although a positive biochemical result can substantiate a clinical diagnosis (muscle histochemistry or respiratory chain complex analysis), the results are often inconclusive. Subtle histochemical defects can occur as part of healthy aging, and individuals deconditioned from any cause can have low respiratory chain enzyme activities in skeletal muscle. However, pursuing a genetic diagnosis is important because similar clinical and biochemical phenotypes can be sporadic, maternally inherited (through mitochondrial DNA [mtDNA]), autosomal dominant, autosomal recessive, and occasionally X-linked. Thus, defining the underlying gene defect can sway the recurrence risks from zero to very high and everything in between. Targeted genetic analysis typically proceeds on a step-by-step basis, guided by the clinical picture and the biochemical profile. A thorough laboratory workup takes months or years and is still inconclusive in approximately one-third of cases. If ever there was a need for a genetic revolution, mitochondrial disorders have a strong case. The first genetically defined mitochondrial disorders were identified in the late 1980s and early 1990s. Being only 16.5 kb, mtDNA analysis was experimentally tractable, and the next decade saw major progress. Mitochondrial DNA deletions and point mutations were defined in patients with a growing array of phenotypes, leading to the first epidemiological studies. In adults, mtDNA mutations emerged as a common cause of inherited neurological disease (1:11 000). In children, nuclear gene defects seemed to predominate but were very challenging to define at the molecular level. Progress in defining nuclear-genetic mitochondrial disorders was slow and limited by the available technology of positional cloning, chromosomal transfer, and candidate gene analysis. This changed dramatically in the late 2000s, with the advent of massively parallel sequencing, enabling the rapid, cost-effective screening of hundreds of genes simultaneously. In the early stages, the greatest success was seen in patients with a presumed autosomal recessive disorder and a defined biochemical defect affecting a single respiratory chain enzyme complex. This pointed toward homozygous or compound heterozygous mutations in a known list of candidate genes that could be captured in a multigene panel. Advances in understanding the protein composition of the mitochondrion (the mitochondrial proteome) led to the development of mitochondria– specific capture arrays.1 This helped to identify new disorders, but progress was hampered by limitations in the efficiency of the capture system (which isolated segments of genomic DNA of interest) and what now seems like a rudimentary understanding of genetic variation in the population. Growing population genetic sequence databases have revealed the extraordinary genetic diversity in healthy humans, but with tumbling sequencing costs and near-complete (>98%) coverage of all known protein coding regions (the exome, approximately 1% of the entire human genome), exome sequencing became the platform of choice by 2012. Exome sequencing dramatically improved our ability to diagnose mitochondrial disorders at the genetic level. A particularly challenging group to diagnose was patients with multiple respiratory chain complex defects who did not have an underlying mtDNA defect. In vitro translation studies implicated a defect of intramitochondrial protein synthesis thought to have a nuclear genetic basis. The limited number of candidate genes meant that, even in 2011, it was only possible to make molecular diagnosis in less than 5% of patients.2 However, exome sequencing immediately increased the diagnosOpinion