The other human genome

In the past 13 years, a new chapter of human genetics, "mitochondrial genet ics", has opened up and is becoming increasingly important in differential diagnosis. Although the clinical manifestations of disorders related to mit ochondrial DNA (mtDNA) are extremely variable, recent advances in genetic t esting aid in the identification of patients. Muscle morphology can give im portant clues for diagnosis, but histological features alone cannot define a specific disorder. Biochemical analysis may reveal a single enzyme defect , or when multiple activities are affected, suggest an mtDNA mutation. Howe ver, definitive diagnosis often requires DNA analysis and documentation of a specific mtDNA abnormality. Disorders associated with mtDNA mutations are associated with a wide variety of syndromes, and owing to the properties a nd characteristics of mtDNA, these are often transmitted by maternal inheri tance. Although therapy for mitochondrial diseases is limited, identificati on of the molecular defect is important for genetic counseling.