Mitochondrial gene therapy. Since their discovery during the end of the 80's the number of diseases found to be assiociated with defects in the mitochondrial genome has grown significantly. Organs affected by mutations in mitochondrial DNA (mtDNA) include in decreasing order of vulnerability the brain, skeletal muscle, heart, kidney and liver. Conventional biochemical treatment having reached an impasse, the exploration of gene therapeutic approaches for patients with mtDNA defects is warranted. For now mitochondrial gene therapy appears to be only theoretical and speculative. Any possibility for gene replacement is dependent on the development of an efficient mitochondrial transfection vector. In this review the current state of development of mitochondria-specific DNA delivery systems and deve­lopment of mitochondrial gene therapy is described.